This is why people need to pick up the phone and talk vs texting, IMO. It sounds to me that that got a gift they plan to split x number of ways and if people bow out, they need other people to fill in.
The way I deal with group gifts is that I don't pony up til I have more info. If they buy something I cannot afford my share of, that puts me in a tight spot. I also don't send money til I'm sure it's a sure thing. One of my relatives has done that a couple of times "oops, we didn't get it" and then I'm scrambling for a gift and have to tell her not to cash the check or send me my money back. This time I waited til they had the TV in hand and gave them my share THEN.
You told her you have no idea, but that's a lie, right? You know he's upset about how it all went down and he feels disrespected. So maybe instead of "I don't know" say, "You should call him and talk to him yourself."
Bottom line is you don't really know her and you're not attending. So she can't dictate what gift you provide, if any. I'd RSVP my regrets and ask not to be included in the gift.