So there's a couple problems with throwing him in with your girl: 1) stress as you mentioned but disease also, and 2) breeding too young which can hurt or kill your girl. They cannot be kept together, the "breeder" mislead you.

I would tell the friend of yours to take him back to the store.... Tell him that you can't afford to keep him and have nowhere to put him. Unless he'd like to buy you a new set-up too? If there is a reptile rescue in your area, that is an option. Although you seem really concerned with making a quick buck off this....

In the meantime, buy a heating pad for your 30g (more than big enough), throw a cereal box hide in there and a water dish, and leave him alone. If it's only for a day or so, you might get away with using (carefully wrapped) heat packs or those mitten warmer things...

Don't worry about feeding him, he can go a week or two until you get everything sorted out. Adult mice and rat pups are very comparable in size, and he may switch to f/t after he settles in (be prepared, that may involve feeding live mice for a couple meals).