Opinions needed! Possible cheater in friends marriage

My friend is going through yet another rough patch with her husband. I've gave sound advice in the past but this time I'm confused! Here are the details: they've been together 18 years. He's cheated at the fist start of relationship (few months into relationship) they got over that mess. 3 years into relationship broke up (he slept with 3 women 1 month time span but were broken up) got back together and pregnate a month later... After few years many friends say he's in bars and such. Fast forward many stories... He's been caught red handed in bar and lusting after women. at a party a person made a comment where a prostitute was conducting business, she caught him in the parking lot (it was a gas station,so she felt doubtful) she's asked him for lie detector test and he flat out refuses!

Now I've explain in the past, start of a new relationship mistakes, the fact they were broken up and maybe coffee at gas station (althought he heard news about prostutite few hours prior.... I tried to make her see all sides first before jumping into a decision.... But this lie detector test... He has a history and with the doubt... Wouldn't he want to prove innocence? I'd just like some opinions before I weigh in.... I'm clueless! And of course this hurts because I do think the worst! :) thanks for any input! Oh!!! They are married 7 years I think...