Since she's locked up in the hospital I've been wondering if Dr. Hopper is actually treating her and that maybe he will somehow become aware of who she is through his interactions with Mr. Gold when he begins treating him. Not necessarily their FTL history together – I don't think that Gold is going to talk about the fact he is Rumple and was in love with Belle, but if he talks about a woman he loved who died . . .who knows – Archie might put it together and figure out that the woman in the hospital he had been treating is the woman Gold thought was dead. He isn't too keen on Regina anymore so he might figure out that she was involved in her being locked up in the first place.
So based on the above, his reaction could go two ways. If he is told about her by someone, he could end up being brought along by someone to rescue her and bring her out of the hospital. If he just suddenly sees her – I don’t think he is suddenly going to go all cane-mad Mr. Gold again. I think the more human/less evil moments we have seen of Rumple were when Belle was with him so I would predict he would almost be so shocked to see her that his evil Goldness would be momentarily shocked away and I don’t think he will know how to react – I can see him kind of withdrawing from the situation, just like he did in FTL. I don’t think he will know how to react when he realizes she is there and he can be with her. I kind of predict he is going to be afraid, once again, that she would never really be able to love him and that, even if she does remember, it is going to take a lot for him to believe that someone could really love him – even in Mr. Gold form. If she is messed up from being in the hospital, I tend to agree with a lot of the fanfic that I can see him wanting to be the one to provide a home for her while she assimilates to Storybrooke and so he can assess whether she remembers anything.
But definately, it is going to give him a whole new incentive to beat Regina at her game!!!