The trick is to get just get your girlfriend to enjoy working out or at least workout regularly. Once you do that her body will keep wanting to work out and she will be making ALLL KINDZ OF GAINNNZ (in this case losing weight). The weight isn't going to lose itself so make sure the diet is proper and all that and u will be set. Weight protein isn't necessary, and I would sub the carbs for clean healthy food rather than spending money on shakes.
I'd recommend watching some of elliot hulse's and scooby's videos on working out. Not only will you learn tons but you'll be on the path in becoming the strongest version of yourself.