Most Helpful Guy
There really isn't much you can do unfortunately, if you are really close to her you can tell her how you feel about it and tell her that what she's doing is wrong, but its ultimately her decision. Yes it is cheating, and the fact that your friend doesn't care, and wants to do this even though the guy has a family speaks volumes about her. It is completely wrong and its honestly fucking disgusting. The guy is 27, I really doubt he would leave his kids to date an 18 year old, so if your friend is expecting anything serious then she will most likely get hurt, but she is pretty much his mistress and if she is fine with that then I guess she won't get hurt.
Most Helpful Girl
My friend is currently dating a guy who treats her like crap all the time and has cheated on her multiple times. As much as you want to intervene and say hey this guy is a jerk don't go out with him. Your friend has to make this decision herself and learn things the hard way. It's sad to say this but there's nothing you can do to help her. This is her choice and if she makes the wrong choice she'll have to learn from it.