Ethics and poly.... ohhhh ethics and poly.
Humans put their own needs first - it is the nature of the beast. Even the kindest, most wonderful, most empathetic and giving people do it. Don't get me started on the philosophical debate of altruism.
What you are essentially saying here, to us and yourself, is that you actually realise you would be ok if her marriage ended, because there's a little part of you that sees the good in that - for her and for yourself.
So, what you are actually doing, if you look deep, deep down... is that you are facilitating her behaviour. You are facilitating it because you fill a friendship need, you help her fill a friendship need, you keep the line of communication open in case they do end and you keep your foot in the door by playing a part in a situation.
Personally, I worry about people that lie, on any level. It's the oldest cliche out there, but I've seen it so many times. For example, my girlfriend's secondary is a married man. They have never met in real life (this is how he convinces himself they aren't having an affair) - but they've had an online, and on the phone, emotional affair for two full years. He lies to his wife every day about this. Over the last few months, my girlfriend finally came face to face with his lying, herself. He met someone else online and kept it from her. When she found out, it was incredibly painful and they broke up. They got back together, but even now, she's struggling with his honesty.
I tell you this anecdote because I know her secondary very well - he is a close friend of mine. He is a wonderful, very selfless, sweet, peaceful, kind soul. But man, he cannot tell the truth to save his life. My GF facilitates his behaviour. She justifies her part in him cheating on his wife, by saying that his wife is selfish, doesn't fulfill his needs, is close-minded, is sexually frigid, unfair, etc etc.
Sometimes people are stuck in shitty marriages with genuinely shitty people. If we enable their cheating, or their dishonesty, we aren't actually helping them grow. We are helping them stay in a bad situation. I personally really tire of the idea of staying together for children. It's 2013. I do not understand this concept. I understand an agreed marriage of convenience, to keep kiddies and parents under the same roof, with agreements on discrete dating freedom. But I do not understand lying.
Ultimately, you will make the decision that suits you best. I totally understand it. I've been there - I've been with a married woman in the past and I've talked online to married women more recently. If it doesn't make you feel bad, guilty, yucky or immoral... then it doesn't. Ultimately, as long as you can accept that you are enabling her to stay in a situation that probably won't change... and as long as you are ok with any potential consequences... then you can continue easily.
Should you be friends? If you're 100% moral, no you shouldn't. You shouldn't facilitate lies, you shouldn't facilitate unhealthy behaviour - in yourself, for your own wife, or for her, or her husband, or their little kiddies. Incidentally, on the friendship front.... can you be sure this will not turn into something more again?
Me: (30f) open poly
GF: (40f) My long-term, long-distance partner
Hubby (37m): GF's husband
Garcon (26m): GF's submissive/third partner
“Peace comes from within. Do not seek it without." ~ Buddha