He fell for lies and pranks
That sounds quite serious. What kind of lies did she make up?
She lied about being pregnant, she said it's my baby, I believed her until the 3rd month then she admitted that I was not the father of the child.
That sounds like a big lie. It is important to always think critically especially if the source is a known liar.
He should have thought about it beforehand and not get himself into this situation.
Yeah, it's really difficult to get involved with such a person, and if they're believable or not. How about trying to talk to him to let him see how she truly is? Don't you want him to be in a happier relationship?