No proof means, cant do anything, such money should have been given in way of gift cheque, so could be claimed later if marriage does not work. Papers will be in boy's name, so no question of getting it back, while coming out of matrimonial house, girl could have taken keys of car, drove it out.
BTW, what interest do you have here? Dont interfere in girls life ever, you may end up in jail. Be careful.