If my gf and her husband split up I would be devestated. Now granted, I'm in a different situation because while we're in a vee relationship I have feelings for both of them. Moreover, I think their relationship is a really beautiful thing.
But those factors aside, I really don't know if things could continue with my gf and I if her husband was out of the picture. She would be an emotional wreck and I would do my utmost to support her, but there'd be no way I could fill the void she'd be feeling. Not because I'm not a good lover or partner, but simply because the two of them have been together for ages and made a life commitment. How could she want to be intimate with anyone when feeling as terrible as she would inevitably feel? I could easily picture her just closing herself off from the world... including me.
Let's say she *did* manage to get over it somehow. Would we be ok then? Again, I truly don't know. Our relationship would almost certainly change. We see each other fairly infrequently now because we're both very busy. Her schedule and life would change drastically, and presumably we'd have more time for each other. Which, in theory, could be good for us but would just be a big change. I'd rather that we take things gradually and let it build naturally as opposed to suddenly finding ourselves in a completely different situation.
So, that's my take as a secondary -- I am all in favor of my partner's primary relationship and don't want it to go anywhere!