Can't say. When is the wedding? Has she already incurred massive expenses for things that can't be changed?
It's none of my business, but I think promising $30,000 is crazy. Unless you just happened to be comfortable enough that you have it sitting around in CASH for that specified purpose and it doesn't dip into retirement accounts or anything.
You (all of you) need to sit down and discuss this like adults. She's getting married and she and her soon to be spouse are adults. Time to act like it. If something happens to put a wrench in plans, she and husband are going to have to deal with it as adults in, so now's as good a time to start as any.
Hopefully, you didn't raise a spoiled brat who will make outrageous demands lest you feel her wrath. And that you did raise a young lady with some sense and compassion, who will understand that everyone's situation has just changed (not just her wedding situation) and that in the big scheme of things, $30k for a wedding is a want not a need. (assuming the wedding isn't in 3 weeks and everything is committed financially already.)
You didn't mention how long you anticipate before your husband will find work again, or if it might or might not be similar in earnings capability. That's a factor, too, I think.
Going forward, it would probably be wise not to commit to large financial gifts to people if you don't have the cash sitting there to hand them.
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ETA
Since you added/changed your post to include some additional thoughts, let me also add this:
It doesn't sound like you were on board with promising the 30k from the start. What happened there? Is this his child from a prior relationship, or are you and husband just that far apart on an issue like this? When did he promise this to her, and why didn't you say something THEN? I totally agree with you, btw. But it's a little late in the game to say, "oh, oops, we aren't giving you any money after we promised a ginormous amount" if their wedding is scheduled in the next few months. I get that losing his job may have brought it to a head, but that begs the question... do they have a wedding date planned or not? If there is no wedding date on the calendar, and nothing has been reserved or contracted... then he goes and talks to her and tells her, gee honey, I just can't afford it. And she behaves like a grown up. The end.
Which, sounds like, you would have wanted him to do even before he lost his job. So... what was that conversation like? You did discuss this promise at the time, right?