Try and think through problems like these logically. If you think or know someone is cheating on someone, then tell them. Don't tell them outright that this guy's a douche bad even if you know its true. Try and make it seem like that you suspect something but you hope you're wrong. You owe at least that much to a person. What they do with that info is their business and their business alone. Maybe they already know, maybe they'll get burned later on in life. But at least you've done your duty.
Now if you're asking about advice on a more spiritual level, like how can people do bad things without feeling any remorse and stuff like that - then you're out of luck. Life sucks for everyone. Some people are actually good people, they just don't realize that what they're doing is wrong and that it's something they should feel bad for. When my sis was younger, her and her friends would skip class and go and kick at an ESL class's door and make a run for it. After school, she'd come and tell me expecting me to laugh along with her. I of course shouted at her and made her feel like shit. She's not a bad person, she just didn't realize how wrong her actions were. She felt really bad afterwards.
Then there's some people that just don't care. They don't care that cheating on someone will hurt their partner. These people usually have a twisted sense of what's ok and what's not. And I don't mean this in a criminal sense (like they're not murderers or rapists or anything), they probably just think 'hey if you're stupid enough to be cheated on, you deserve to be cheated on'. Or this is my life, I'll do whatever I want.
Sorry for the long post and I hope this helps. I'm an atheist, and I don't actually know much about Christianity, but if you're a true believer than believe in that. Believe in your god. Also this is just my opinion
. I'm sorry for what you're going through and hopefully you'll get past this problem soon.