[2step]

It could be that the coolent was quite low in the first place and you haven't added enough. The heater not working is a sign of the coolent being low.

If you added coolent without the engine running and everything being warmed up with the thermostat being open, then you haven't displaced all the air in the system. Let the car run for a while, with the cap off, until it's near operating temp and the waterpump is circulating the coolent and then top it off. That way the air and steam will all be pushed out and coolent can be filled to the proper level. The fresh coolent needs to be slowly blended into hot stuff so as not to cold shock any hot engine parts and cause them to crack.

Also pour additional coolent in the overflow resevour because the coolent will shrink and contract as it cools and pull the extra from the resevour instead of pulling air.

Be very careful opening the radiator cap when the coolent is hot. It can immediatly turn to high pressure steam and burn you badly. It can be done if you are careful to insulate your hand with something like a large towel and you only turn the cap to the first detent. The towel should be generous enough to screen all bystanders from jets of hot steam. The hot steam has to go somewhere and if not on you then all over your engine. I don't recommend that anyone try this unless they really understand the forces they are dealing with.

I don't know why your girlfriend's coolent got low to start with. It may be a problem but it's common to just be able to top it off and your done.