You really just have to make a decision about what's really worth it to you, and what odds it's worth gambling on. I can see the rationalization for the cheating and I don't even strongly object to it, I'm just saying that a cheating situation carried on indefinitely (no exit strategy) is likely to come to light sooner or later (i.e. you're likely to get caught). So that would be an additional headache you'd have to cope with (on top of the heartache). I don't mean to tell you how you should live your life but I do advise a whole lot of caution (and some kind of exit strategy perhaps).
I guess when I say exit strategy I mean a point in time when the husband here would make peace with you and his wife being in love (emotionally committed) and openly permit the two of you to continue having sex with each other in spite of the emotional entanglement. That and, some kind of timetable as to the longest amount of time you're willing to risk the having sex in secret while waiting for the husband to come around.
It must complicate things when they're returning to their home country soon? Will you try to keep the relationship going at that point? an LDR, perhaps? Will you do a lot of traveling to visit her? Just curious, ignore any/all questions where I'm getting too snoopy.
I kind of get the idea that she'll probably divorce him eventually? then hopefully move in with you? something to that effect. I guess it's possible. Would/will you indeed want that, I guess is my only remaining question.
Sounds like a messed-up situation; I hope it works out alright.
Love means never having to say, "Put down that meat cleaver!"