I think DigitalPumpkin and others have made great suggestions. I wouldn't approach it as he's been duplicitous or I'll only go to dinner with you if I'm your GF, instead just let it go. It takes time to realize that someone is right to marry and it sounds like they both invested the time with good intentions and that when he knew it wouldn't go in a romantic direction he told her. Similarly, the I'll say yes if I'm your GF seems like it encourages hiim to misrepresent what's happening in the relationship. Instead, just saying, I'm sorry you feel that way, but I want to devote my time to meeting someone who I'll eventually marry and since that won't be the case here, I must decline is more fair.
I must say, the combination of saying I could see us happily married, but I don't want the relationship to become romantic seems very conflicted. I'm not sure how I'd interpret it. In other circumstances I would think it meant I don't want to become physical until after marriage, but doing so is inconceivable in this setting.
One alternative is for her to be equally candid and say that he is sending a very mixed message, is he saying he wants to marry her but not have a romantic relationship or that he doesn't want to marry her or that he wants to marry her in the future, but just not now. If he is just confused, she might suggest that he talk to the rabbi to help him clarify his feelings and then proceed from there.