How can I write a big betrayal in a relationship?

I'm writing a soapy romance story and the relationship is going good but the girlfriend does something so horrible that the boyfriend can't forgive her and it's even way bigger than cheating but I am having trouble deciding what that betrayal should be. Now she is really sorry and begs for forgiveness but what she did is very hard to forgive. Other than cheating what's something so horrible and such a betrayal in trust that would end in a break up? Thank you for your help and suggestions.