I’d recommend going to your local parts store (or probably Amazon) and getting UV dye specifically for the cooling system. You dump it in, run the car for a bit, then start looking with a black light (you’ll probably need to buy one of those too). The ones we use flash a bright yellow/green where the leak is originating from.
If that’s not something you’re comfortable with, call your trusted mechanic and see if they can do it.
If you’re losing a considerable amount of coolant, don’t let it go, you could be making a relatively simple repair significantly more expensive, or possibly even killing the engine, if it’s as bad as it sounds.
Also, has the vehicle ever been overheated before, or is it getting hot when she drives it?