Had a similar issue with my wife, she's a bit of shopaholic for 'small' purchases, it's not like she rushes out and buys a £500 handbag but she'll frequently buy stuff for £20-50 and it all adds up. A lot of what she'll buy is just things for the kids, like if my 3 year old shows an interest in something, she'll invariably buy something related to it. We've got shelves full of books, toys, puzzles, learning materials etc that are barely used. DVDs sat in their wrappers for years etc. I think she gets some sort of dopamine hit from "it makes sense to buy this item" and then buying it, rather than actually using the item (I even do this a bit like a Ring doorbell last Black Friday that's not been plugged in yet, but I certainly don't get myself into debt over it). Even stuff like food shopping, she'll buy too much and we end up binning at least a tenners worth most weeks. She's very much a fritterer but probably wastes hundreds of pounds every month when you add it all up.
The thing that annoyed me the most wasn't so much that she was in debt but more that she was wasting so much money on interest due to paying min balances and stuff, whereas I had the cash sat around. So if I had known earlier I could have paid it off earlier and we'd have avoided all that wasted money. I only found out when she stopped being able to pay the normal amount into the joint account each month and I quizzed her about it.
I don't know what the current situation is, she earns more than she used to but she also leaves letters from credit card companies unopened for months, to be fair it's all online these days but it wouldn't surprise me if she's racked up some level of card debt again.
As for the OP my advice would be not to fly off the handle about it, just act rationally and sit down and say you want to know the full extent of the debt, that £6k might be an underestimate / rounded down, she might have other debts she's not mentioned. If you act too angry there's a risk she'll hide things from you in future. Just be calm and clear that you need full transparency, you'll be there for her but you need her to be totally in the open to maintain that trust.