My son is about to come into some money. If he were to split up with his girlfriend, with whom he shares a house, could she claim any of it?
No. Cohabitees, unlike married couples, currently have no duty to maintain each other after they split up and have no claim on each other’s assets or money, as long as it’s kept separate. The only circumstances in which the girlfriend could lay claim to your son’s money (unless he’d promised to split it with her) would be if he were to die after they had been living together for two years. If he had failed to make provision for her she could go to court to claim part of his estate, whether or not he had been maintaining her financially.