She will keep hugging and kissing them on the lips. My mother is mid-stage Alzheimer's. Whenever she is in the presence of another male besides my Dad, she will keep going up to them and hugging them hard, and kissing them on the lips and telling them she loves them. My sister recently had to pull her away from her boyfriend. We can't reason with her at this stage, so don't know what to do.
But that's not to say it is an easy one. It will be difficult to deter him without hurting his feelings and destroying the caregiver-client relationship, too. She needs training in this tricky area. Bring it up with her, but do it sympathetically. Do you know the correct techniques, by the way? 'Cos I don't - if it were me, I'd be looking for a specialist course (or consulting Teepa Snow).
I don't know if anyway to stop her from doing it. From my experience with dementia patients, they will not remember that you asked them to refrain from kissing the men. I could explain something a dozen times, but my cousin would not remember it. Even if you write it down, it's not likely she will remember to read it.
It's not that she is willfully trying to be inappropriate, her brain is not functioning the way it used to. It may cause a loss of inibitions. I would be glad that she's not paranoid or violent. Those things can happen too.
From my experience, the quirky behavior goes in phases. This kissing phase may or may not last. Sadly, once she progresses to not being able to walk, she won't be able to approach the men to kiss them.
I know it's the school of thought that I come from, but (after demonstrating as much tolerance and understanding as possible) I don't believe that the rights or feelings of someone with dementia should trump those of the people around them.