You're definitely right when you say it's a messed-up situation... it has really only persisted this long because of my obsession with her. Thing is, it's not like your normal triangle because he knows me, and likes me as a friend (sounds weird but is true). We never discuss her, mainly because he is scared to do so (he's a bit of an ostrich about the whole thing and doesn't want any confrontations).
And as for 'getting caught', he knows it has been going on and could have intervened on pretty much any occasion, but has avoided it.
Anyway, for now sex is off the agenda and I am looking elsewhere - I have a feeling that when things get easier for her (less time pressure with her studies, and him away from home) then we'll be back at it again, if I haven't found someone else in the interim. She may be somebody I end up with in a few years time when their son has grown up and they divorce... whatever.
Thanks for your input on this, and I think I will just count my lucky stars for now for some of the best times and the best sex I've had in my life