Hi Frances. Sexual behavior in any alternative living situation such as assisted living or nursing home is very difficult to handle.
First, anyone who is considering placement in a facility
should ask if for a sexual conduct policy. Many do not have this, but the word is catching on that this is totally necessary. The policy is not just for you, the caregiver, but also for the residents. So first ask for this policy and see how they handle situations like this.
Second, it is difficult to inhibit sexual behaviors, even in cognitively intact human beings. For those with dementia, the concern is augmented because you don't know what is true or untrue and the parties involved likely do not remember and cannot control these behaviors. They may even make up stories to "cover" their lack of memory. The other difficult issue with dementia persons involved in this behavior is whether or not the sexual act is consensual. In many cases, the dementia person does not know what is happening but "feels" something that is good. It's always easy to blame the other person.
Lastly, medication likely is not the answer and could make things even worse. I certainly would not want to do this for me, my loved one, or the "accused" party. Libido is natural; but for dementia persons, it may be difficult to control due to lack of knowledge, consent, or disinhibition.
At the end of the day, I would ask for a secual conduct policy and try not to "dwell" on the issue too much. The act has been "done" and you can't turn back time. Also, moving on may help any associated anxiety even though it does not take the pain, violation, or act of infidelity away.
UPDATE: I want to clarify the issue of "dwelling." Your mother is going through a significant sense of loss and this experience only aggravates that loss. Please acknowledge this. It's a difficult thing to forget. If you put it in a context of loss and grief (not directly to her), then you may be able to heal and understand his position. It's OK to be sad and disappointed.