People always say "Once a cheat, always a cheat"
You need to dig much deeper than that.
In my 1st marriage, my husband cheated on me several times, with several women. He tried to have sex with my friends. He did actually shag my best friend. He was also quite mean and belittling, didn't help around the house, did no grunt work, he would kick me sometimes out of sight of others, and assaulted my twice. I eventually had what I would describe as a mini breakdown. 4 years after I found out about the affairs, I myself cheated on him. I didn't go looking for an affair, I was in a bar one night, had been chatting to a guy, and he suddenly went in for a kiss, which was unexpected, but went through me like a bolt of lightening, waking me up to the horror of my marriage.
Me and first husband split shortly after that (I left him). He has gone on to have 2 serious relationships after me, and has cheated on them both.
I, on the other hand, have been happily married to DH for 16 years, and I have never once cheated on him, and never would.
So, once a cheater, always a cheater, is not true for me at all. It's not in my nature, but I still did it when I was in a terrible marriage. My Ex on the other hand, it would seem to be in his DNA.
Only Op knows her partner and which camp he's likely to be in.