I've been trying really hard to look at scrapbooking from the 'what's in it for me angle' and I have a little advice for other partners of scrapbookers.
Indulge the hobby.
If they aren't serious, sure you will have blown a lot of money, but then you can feel free to take up and dump and expensive hobby of your own without a huge guilt factor.
If it turns out that scrapbooking is a serious obsession then there is not much you could have done anyway. Keep the family scrapper happy. Keep the supplies of paper and product coming and establish a scrapping budget. At the same time establish your own hobby budget - even if you don't have a hobby the money will come in handy when Dana Darling or some other fabulous scrapping guru releases the latest in fashionised camera bags that are the 'must have' of the scrapping world.
Mrs Grendel loves hers. . .