@Eralione Lets be honest, you issue isn't about the marriage its about the potential peril you see in regard to the monies you have gifted???
This is not an impulsive marriage, its been on the cards for ages and from the sounds of it you like your future DIL. They’ve been together nearly a decade, they’ve already weathered a breakup and got back together, both employed adults, they’ve bought a house and are already functioning as a committed couple. This is not an impulsive teenage romance. They’re basically a long‑term couple formalising what already exists.
You have a legitimate concern. You’re worried because the house was bought outright, the money came via a deed of variation, the house is (likely) in your son’s sole name, marriage changes the legal landscape & divorce could expose that asset.
Once married the house becomes a matrimonial asset, even if it’s in his sole name, even if she didn’t contribute financially, even if the money came from inheritance and Courts look at needs, fairness, length of relationship and contributions (financial and non‑financial).
Given they’ve been together since 14/15, a court would treat this as a long relationship, not a short marriage. So yes, the financial risk is real.
But here is the thing, you should have considered this when you gifted the monies as in all likelihood he was going to marry at some point, maybe not to her and this was always going to apply and I find it strange that given the length of time they have been together, this is now concerning you, it should be reassuring you UNLESS there is something you are not telling us?
In reality they have been together long enough that to all intents and purposes they live as a married couple and now they are just making it legal.
There is a solution for this should your son so wish and assuming the house is only in his name - a prenup. It protects the house, the inheritance, your son, protects her too, avoids future conflict, is increasingly common and is legally persuasive if done properly. But its also very unromantic and who knows how she will take it because if they intend to have kids it could change the landscape of her and the kids financial security should they separate unless that is covered in the prenup.