Uncomposted Cow Manure
We assume you are speaking about his vegetable garden, right? How large is his garden?
In normal sized vegetable gardens, this is not something that we would recommend. There are concerns about drawing flies/insects, but most importantly the possible run-off of water contaminated with pathogens that could cause illness.
This would be especially true if he were growing leafy greens or other vegetables that are difficult to clean and are not peeled.
It should be mixed into the top 4-6 inches of soil;
Here is our publication on soil amendments with more information: