friend cheating on boyfriend

Ugh I just feel bad, but I'm staying out of it. But this is the 3rd time my friend has cheated on her boyfriend(with the same guy). She's been with her boyfriend for almost 10 years and they have a child together. He cheated on her 3 years into their relationship and she found out about 3 years ago. But he only did that one time(not that its okay). So I think that's part of the reason, and another is because they don't have Sex as often as they use to. And I know there's other relationship issues. But the shitty thing is. Is they're suppose to be getting married next year. Which is part of the reason she cheated in the first place is because after 9 years she was tired of not being married. But he finally proposed about a month ago. But just recently she slept with this guy again. It just bugs me she's doing this when they're getting married, have a kid together. And it's like a lot of the problems they have they really just shouldn't even be together. Like I get her reasons, but I have some of the same issues with my boyfriend(who is best friends with hers) especially the sex thing which I get it way less than she does.. but I have no desire to cheat on him, and he has no desire to cheat on me or anything. Idk I just feel bad and i just don't understand how she's okay with doing this 3 times...