Tricky, because the world is full of people making bad decisions, and you can't go around telling all of them.
Lots of people are very poor and the kids can grow up to surpass all expectation. Telling people they can't afford to have kids is sort of morally not OK, it's a basic human right for better or for worse. I know uppety fully medically insured (through a spouses work, not on their own dime) wealthy people with kids in expensive neighborhoods going to private schools who are in LOADS of credit card debt. Who are they to judge people with far LESS debt using a system they have also paid into to help have a family? Are those kids genetically inferior? And not all poor people use the system. My husband an I paid for 3 child births out of pocket and we're NOT rich, but we deserve a family as much as the next guy and we provide very well for the kids on every level with no assistance and no debt by living modestly.
Certain wealthy friends of ours "suggested" we ought not to any have kids if my husband's job didn't offer insurance. Really? Because in America things aren't structured to support all parents, we should plan our lives and futures accordingly? Talk about letting the government run your life! This advice came from an entitled wealthy wife who has NEVER worked a day in her life who felt no one poor should have kids because THEY'RE irresponsible. We now enjoy 3 kids while they struggle with one spoiled one.
However, you say he's a cheating cad, and she's trying to trap him by having the child, and they may make horrible parents. Again, is this any of your business? Maybe. If she's a real friend to you, and you are speaking from kindness, share your thoughts out of genuine concern, not judgment. If she hates you for it, she probably wasn't your real friend.
That's all you can do.