Use £4k of her money to pay off her own debts. Leaving 6k
Give £4k back to his parents. leaving 2k A regular income may be as good for them as a lump sum - need to ask them what they want to do.
That leaves £12k in the kitty - six from each side.
Get the woman to stop being a dick with her money or it'll end up with them both in a right mess. I've seen this so many times, one half of a couple is fucking financially clueless and/or irresponsible and years down the line they find they owe as much, if not more, than what they did when they started off fifteen years earlier. All they have to show is wardrobes full of shoes and clothes that in reality are out of their price range and cupboards full of crap they've never used or are likely to.
He needs to get her sorted and watch what she's up to.