Heat exhaustion is a very common experience. Many people will have this during the summer months. However, if there are concerns that this is happening "too quickly" or is too severe, I would recommend that you see your primary care doctor
Heat exhaustion is a symptoms of essentially the body's "engine overheating." Your body has many symptoms in order maintain a normal systems - or homeostasis. Maintaining a normal temperature (eu-thermia), is a closely regulated process. The normal body temperature is 98.6. There are many ways the body works to get rid of excess heat -- sweating is one of the most prominent, but there are others. Each person may have either better or worse systems to get rid of the excess heat.
It sounds like your girlfriend does not have a good set of systems to get rid of excess heat. This can cause heat exhaustion. You are right that dehydration is a component of heat exhaustion -- and if it becomes severe can cause "heat stroke" or fainting
caused by dehydration and heat. Therefore drinking cold fluids is important. Unfortunately, this is only part of it. Simply avoiding heat and especially direct sunlight (stay in the shade) is the most important.
Again, please talk to a doctor for more information. Good luck!