I have added a few more pictures to reference. Thank you!

Here are two more photos for reference. Thank you again for your help!

Take care,

Courtney 

Hello!  Happy to help.

I'm contemplating several possible issues and slowly ruling out some.  For instance, since the tree's been doing fine until now, you're probably providing it with at least 6 hours of sun per day or at least supplemental light in the winter.  So I don't think it's the lower light of this past fall that's been the problem.  But it can be sometimes.  I have had tropical hibiscuses that I've brought inside for the winter drop their leaves like that but they're slightly deciduous so I know they'll grow more leaves starting late February when the days get longer.

What I do see are boots behind your son which tells me there may be a blast of cold as people enter and leave the house through those doors.  That may be a problem but then I see the radiator over to the right which could help mitigate the problem.  I'd watch how cold the plant gets being next to those doors.

Here's what I don't see, how much water are you giving it and are you letting the water completely drain out of the bottom of the pot?  Citrus trees do need water but prefer their soil to be on the dry side.  We recommend sticking your finger into the potting mix up to the second knuckle to check if it needs to be watered.  If the bottom of the pot is sitting in water, that can cause the roots to rot. 

The tree itself does need humidity and the door with the radiator may be making the area too dry.  Some suggestions for that include putting the pot on top of a brick that's sitting on top of some pebbles in a tray full of water.  The evaporation of the water can provide some moisture to the tree.  Another option is crowding other plants around the lemon tree and putting a bowl of water in the center of the group to provide moisture for all of them in their micro-climate.  As the other plants transpire more moisture is added to the mix.

Other unseen issues may the pH of the soil.  It should be slightly acidic - between 6 and 7.  If the pH is not right, the tree will be unable to take up certain nutrients from the potting soil no matter how much fertilizing you do.  You could send in a sample of the potting soil to be tested by the U of MN Extension if you want to get the lowdown on that.  Be sure to let them know the sample is from your potted lemon tree - they need to know the purpose of the soil to make recommendations.

We usually recommend repotting in the spring just when most plants have a
burst of energy especially with the longer sunlit days; but maybe it would be easier to repot now with a potting mix specially formulated for citrus trees.  We're starting to get those longer days so being a little early won't hurt.

Finally, I'm wondering about the brownish watermark on some of those leaves.  Whatever is causing that may be the same thing causing the spots on the lemons.  I also see that the leaves are dropping while they're green.  I've sent a question about that to a U of MN Extension Ag expert and I should hear back within a couple of days.  So I'll send you another response with her thoughts on this.  In the meantime, here is some excellent citrus growing care information that may be of interest to you.

I hope this information helps and I'll write soon.

Hello!

Lemon trees are not really in the purview of Minnesota agriculture but they are in U of MN Extension master gardener and extension educator Julie Weisenhorn.  Here's her expert advice:

They need a lot of light, temperature changes from day to night, acidic soil, pruning, and they take up a lot of room. Citrus typically bloom in
fall / winter and produce fruit. Putting the plant outdoors in the summer would help it considerably. Growing Citrus indoors

Light is the most limiting factor for indoor plants especially those that flower / fruit. The leaf drop is likely due from a combination of moving the plant to a different location indoors and wet soil. Citrus also use a lot of resources to bloom and fruit and will sometimes drop leaves. Adding a grow light would help the plant to regrow more leaves. Lighting for indoor plants and starting seeds [Also, m]oving the plant to a safer location away from the door would eliminate the draft issue.

I recommend the owner feel the soil - probes are only reliable if the soil moisture is tested in numerous places. I see the plant is also growing in a pot set in a decorative basket. The inner pot should drain well when watered. The plant also shouldn't sit in excess water. If the owner found the plant was drying out quickly (and thus she watered it often), it may be potbound and in need of repotting to a larger pot. She should wait until the plant is done fruiting to do this.

The plant is actively growing, so the owner should be fertilizing the plant regularly at half-strength with a product specifically for citrus trees or a product for acid-loving plants like Muracid (easily found at garden centers / online). I would also recommend watering with reverse osmosis water (R-O) or distilled water. According to the CDC, city tap water is treated with disinfectants like chlorine or chloramine, and fluoride for dental health. Some plants are sensitive to these additives. City water also typically has a higher pH. The owner could buy an additive at brew-n-grow stores that will acidify water, but it's usually easier to just bring home a gallon of R-O water from the grocery store each week and add fertilizer.

Per the webpage on Citrus, they grow well in indoor temps of 65 degrees indoors, dropping 5-10 degrees at night. It would have to be pretty cold in that room or the door left open to damage the leaves.

I  am not sure about the fruit spots. Fruit spots can be caused by various things; however, they are superficial and likely not affecting the fruit quality, so I wouldn't worry too much.

As a tree and shrub care advisor, I would add to wait until the tree produces leaves and finishes with ripening the fruit before making pruning decisions.  That may not happen until March or April or even later considering all the re-leafing that needs to take place.  Check out this link that contains the basics of pruning right at the top of the page.  Feel free to respond to this thread when you're ready to prune if you want to ask me about pruning your specific tree.

It's a lot of reading but I hope it'll help restore your tree.  Good-luck!