Secondly, if their relationship is in trouble you should not stick yourself in the middle of something that might be merely a difficult period for them I think all long term relationships have these. But in your case, you ascribe more meaning to the effects of our more interesting approach than is good for you.
I'm saying it's a bad idea to pursue such a friendship when your head, heart and hormones are looking for more. It's not that simple inside a long term relationship. Trump blamed for Kenya terror attack: Divorcees are likely to divorce again.