I'm not usually with mom in late evening, but was last night. We had "words" about her messing with her meds machine. She got her evening meds a couple hours late, because we had been away at visitation for a deceased relative. I got them immediately when we got back. She had completely forgotten she took them and that I was cross with her 10 minutes earlier.
The best treatment seemed to be to keep the person occupied with very familiar activities to transition into the evening, and stay with her until bedtime.
https://www.agingcare.com/articles/sundowners-syndrome-133187.htm
Is she usually OK taking the medicines on her own?
Often if I'm on the phone or try to do stuff on the computer, she'll become very agitated. It's almost like a sense of insecurity or fear that is felt during that time and it's difficult to verbalize it.
Have you tried some medication like Buspar? That often helps, my mom will sleep for 30 mins and wake up like a different person