I've never been in the precise situation described (which I can't quote
usefully, since it seems to be no longer in my inbox), so I've been taking
my time mustering a reply - it'll be a somewhat generic one, but.
I will, however, put in my thoughts on a somewhat similar situation I've been in, which may be familiar to some people who've been on a list for a while and who caught me in my last 'I need to angst and rant and break things' phase.
A few months ago, I had a long, rambling rant about my incredible frustration with the person who is almost sort of not quite really a secondary partner for me. Basically, he and I have been good friends with a high level of emotional/sexual attraction for about four years, with varying levels of contact over those time (several of those years had moments of, 'I haven't seen J for six months', followed by making contact and going and spending four hours in a Chinese restaurant, just talking).
I'm poly. He has poly tendencies, in a very strictly polyfi style, but is not poly; he has a girlfriend. Seeing them together (when she's not being a... ahem...) they're just so happy I have to grin at them; it's infectious, and I'm not jealous of her per se. He has an open enough realtionship that he can spend time snuggling with me on occasion, for example; however, he's not entirely certain what else he can do without hurting her, and neither he nor I wants to do that. He's shown very little interest in going to the effort of figuring out what the hell the relationship he and I might develop.
After about two years of the past four of me being intermittently led on and then shut down, I've more or less decided, 'hell with this, call me when you grow a clue'. It's not worth the ongoing frustration of the mixed signals. We're still friends, but at a bit more of a distance, so that I don't strangle him in frustration. I don't mind his commitment to his girlfriend. I mind him acting one way, saying something else, changing his behaviour, and then changing back.
I've found overall that it's not really worth pressing the issue with someone who is deciding to be monogamous, for whatever reason, whether they decide that they're not really poly, not intersted in a poly relationship right now, or are respecting the wishes of a mono partner; it's more likely to sow frustration on both sides and put troubles in the potential friendship. (I had a breakup of a poly-relationship some years ago where I just wanted to be certain that I had a chance of something at a later date; I've only just started being able to ahve a conversation with him again within the last couple months.)
People change. Sometimes relationships change. Sometimes people end things, sometimes new things start. Trying to hold on to old things that are dying is a good way of choking the life out of whatever new things are possible. Taking a step back and letting things develop on their own without poking at them with a sharp stick to see what they're doing at the moment is more likely to have a positive result, so long as one doesn't shut the other person out entirely. (At least, that has been my experience.)
I don't think that the 'wait for the other relationship to fall apart and be there to pick up the pieces' school of thought is particularly useful. I personally find that sort of attitude to be somewhat vulture-ish, and if the person on the other end picked up on it, it would almost certainly be poisonous - I'd hate to feel that someone I cared about was so indifferent to my happiness that they were lurking around the edges of my life waiting for my relationship to fall apart and thinking that I would turn to them to sob on. I mean...
I am, however, far too old and jaded for my actual proper age, and my thoughts should be taken with an appropriate grain of salt.
So I guess mostly what my actual advice would be would be - let go. Keep the friendship without turning into a vulture. See what happens. Maybe something will - if so, good. Maybe nothing will happen - try not to get bitter.
Getting bitter is a pain.