When you first said/ wrote he looked at porn, I was like "so what?". I mean, it's not a disease or something, just hormones.
But then you said it was gay porn, and he paid for it (a larger amount of money). That really doesn't saound right. If he was surfing the web for porn and came on such a page and saw it, fine. If he actually pays for it... still fine for him, but not anymore for her.
Yes, he says he isn't gay. Do they live in a region where people are openminded? I can imagine it's not easy to admit you are gay, and especally not if you live somewhere where people are very... narrowminded (is that a word?) I went to the wedding of a gay couple, and one of the two men had been married before with a woman and has two kids out of that marriage. How come? No he hasn't turned into gay after the first wedding, he has always been like that, but because of different circumstances (society, community, family, ...) he just never admitted it to himself and the rest of the world. During his first marriage, the general ideas of society have changed a lot here, and now being gay is accepted by most of the people.
As a friend, I wouldn't directly say "run" or "stay", because in the end, it will come back at you. I think you better give her the advice to go find counceling. Independent counceling, a councelor with open mind, and not someone who is determined to keep them together (chuch is probably not a good idea). They need to find out why he subscribed to the porn site.
Next to that, you can also bring on some questions, like other people here have already said. If she can live with ssomeone from whom she knows he pays to see porn online, then there is no problem.
What I wouldn't do, is tell her to just run away like that. Let them find the "source" for this act, and find a solution/ help/ whatever. Since they are engaged, I can only assume they love each other. That love shouldn't disappear just like that *poof* because of that one thing. In the end, being gay is not a crime. In case he is, and only finds out now, it isn't going to be an easy time for him, and he will need support, and not insults/ anger/ whatever.