Am I the worst? Please be honest

A friend of a friend has been sleeping with this guy for a couple of weeks. They said it was a casual thing but I think she likes him more than she lets on.

A few days ago I slept with him after a heavy night of drinking and after openly flirting with him in front of her.

I know I've hurt her but she says it's fine.

Would you ever be able to forgive someone who did this to you?