I don't think the age itself is the problem. I've seen ppl with 30 year age gaps have happy, fulfilled marriages. However, I think they are just using eachother. It sounds like he doesn't want to die alone and he's so exicted he's met someone who wants to marry him, he's blind to the fact, she's not the right one. And it sounds like she's using him to get out of her parent's house. I've known ppl on both sides of the fence, and this won't end good. So, you can either tell him (respectfully, and lovingly) that you don't think she's the right one for him. Or you can keep your peace and watch a trainwreck. It's up to you. At least if you say something, you'll know you did your part.
Christina