Unfortunately, ummmm, that is exactly what people do do! Claiming a beneficial interest by way of a constructive trust ("we agreed we would own the house but never got round to putting me on the title") or a resulting trust ("I paid for the new kitchen, conservatory etc") is the "new divorce". Whatever the merits of her claim, it could cause a whole load of costs and heartache just disproving her claim. Although a document drawn up acknowledging that on a split "we each take what we came with" will not prevent a claim, it makes it significantly less likely.