OP, sounds like you and your DH have come up with a plan, and good for you; that's the important part, so neither of you gets ambushed later and agrees to something without the other.
I'm kind of torn on the issue, personally. If it was my father--as in, my actual father with all our history, personalities, etc.--I would question him about his plan, ask about cost, ask if he thought my mom really wanted him to spend that much money, etc.. Generally trying to lead him away from the expensive idea or at least discuss it with her first. And if all else failed I would mention it to my mom myself. Normally I'm the type to butt out of things but if the $$ was really getting up there I would see it as crossing the line into seriously bad behavior, and I would not want to sit by and let it happen without trying to do something. In my case I wouldn't even worry about them expecting me to bail them out, because I make/have less than my parents; it would be all about how hurt and angry my mom was going to be.
But, that is how my own family dynamic works, and I have reason to believe it would be successful based on our shared history. I can see how someone else might not feel that way about their own, or especially someone else's, family; and I think if there's doubt, doing nothing, while battening down your own financial hatches, is probably the best option.
The other consideration is how often the OP/her DH intrude between this couple. Of course I would find it tiresome if someone was frequently second-guessing my SO's judgment and tattling to me. But stepping in once, out of genuine concern, over what is perceived to be an extreme situation, is much more forgivable, IMO. Which I guess means I do think stepping in is fundamentally rude, but sometimes the situation trumps that consideration.