If it was a "relationship" : meaning it was exclusive and not an open relationship. I wouls dump them explain why and move on.
Cheating is the ONE thing I WILL NOT tolerate at all.
I desrve better than that and so does wveryone else.

If it was a more casual relationship then it is time to draw rules and boundaries. so a missunderstanding doesn't happen again.

my 2cents hope this helps and if you want to talk about it PM me and I'll help.

I usually end up playing matchmaker and pyschiatrist about relationships

Victoria