I don't want to give my (ex)friend and her boyfriend a chance
What is the reason why you do not want to give them a chance?
Yes, I don't think I can trust them anymore.
It is okay if you do not want to trust them. If you don't want to give them a chance, you should step back from them and not to interfere with their relationship.
It's better if I interfere with it. I can't trust them anymore.
Well it may make more sense to just let them be. If you feel you can not trust them than avoiding them is probably a better option over messing with them. If you choose to interfere with their relationship this will just cause them to want to mess with you more.