Hi, I don't know where to post this so here goes

Today I went on a ride with my girlfriend, a sunny day at 29C temperature hot but not that much. I brought a bottle of gatorade and she brought a bottle of water. We did about 30km before stopping at a convenient store to buy some drinks (we emptied the bottles). I brought a bottle of fresh water to my girlfriend; she started drinking but at some point she started to feel light headed and completly blacked out That is, she didn't see anything and she heard her voice really loud in her head. I brought her inside the store and the clerk brought a wet towel to put on her head to cool her. She felt better and then made her drink an orange juice and eat a chocolate bar. I convinced her to take a gatorade bottle instead of the water bottle afterwards. We did the rest of the way home fine, and she doesn't feel any after effects, just sore legs

She isn't the lazy type at all; she moves a lot at her work and is quite active. But this was only her fifth ride since the beginning of the summer.. I've been commuting to work so have done quite a lot more rides.

What happened?? Were these symptoms of a heat stroke? Was it lack of energy; drinking only water and not getting the electrolytes? Could it have been too tough a workout? (I personally didn't even feel tired after the ride; and she didn't seem out of breath at all! but..?). Just want to know if it happened to some of you guys and/or what resources I can check out so as to avoid this in the future

Next thing I'm buying tomorrow: 2 more water bottles and 2 more bottle supports (one more for each).