Elderberry prefers moist soil and the ongoing drought we've had most of the summer has caused many plants stress, especially those that don't want to dry out like Elderberry. While the watering you provided should help, it can't reverse any damage that developed before that point; leaves can't heal, though new growth should look better if drought was the main contributor to damage.

Overlapping issues may also be present, like spider mite feeding on the leaves being a potential cause of the yellowing. A fungal leaf spot infection might be causing some of the brown spots (earlier drought stress might be causing others), but it does not require treatment. If you are growing Elderberries for harvest, we do have a fairly new web page with cultivation tips that might be useful: Less Common Fruits for a Home Garden. If grown only for wildlife value instead, then less routine care is needed.

For now, just keep monitoring it for watering needs and check for spider mites on the leaf undersides. If they are present or abundant, a strong spray of plain water from a garden hose will knock some of them off (make sure any hot water is emptied first), and if repeated every few days or so for a few cycles. Aim the spray at the underside of the leaves. It's possible the other plant that looks better was less stressed for some reason or happens to have more resistance to leaf spot.

Miri

I checked under the leaves and could not find any evidence of mites so I’ll just be more vigilant with watering.

I checked under the leaves and could not find any evidence of mites so I’ll just be more vigilant with watering.

You're welcome.