They are both responsible for their own behaviour.
He's a slimy cheat. Hopefully his fiancee will find out prewedding.
She slept with a man in a relationship, whatever her reasons for it. She's your friend, how do you see her?
Tbh when I've come across people shagging others in relationships ive noticed 2 things: 1) some tend to cheat too themselves so have no qualms and 2) they are behaving very selfish. Most certainly the latter.
For me, it tells me not to trust this person or be wary as those kind of people haven't proved trustworthy in the past. One i was frends with for years (she did similar to your friend and i should have dropped her after the first few men) - she happily had relationships with men in relationships- and later made a play for 2 close friends husband/boyfriend. She was selfish and had no loyalty to her friends or no compassion/empathy for any of her flings partners.
Another was deeply unhappy in herself and felt terrible about it. She still beats herself up. She is a good person who did something bad, that is how she sees herself. She is a good person and friend. She hasn't ever done it since and had been cheated on before it so knew the pain and beat herself more over it.
In both cases obviously men are 100% blame and shits but in my second friend's case i actually hate him as i feel he manipulated her. He knew she was depressed and felt terrible, he was her friend and he got drinking with her, made her feel like she was someone (ex did number on her) promised her the moon and she fell for it. It was gratifying when his wife left him (not for that affair as she had already known of another) she divorced him and he's left alone and friendless.
So i think it can be hard to answer generically. However anyone shagging soneone in a relationship is behaving selfishly imo, questionably too. But they are not responsible for the person who is cheating and it is true that doubtless the cheat would find another.