I find this to be very true! She found out my summer will be spent taking summer courses from May to August. She isn't liking it and asked me who is more important. School or her. What kind of question is that?! I obviously said school and she got quite pissed.
Dude, this is classic stuff here. First, I wanted to tell you not to COMPLETELY trust what someone is telling you, but to try and verify this information. It's happened before, people are wrong, or just lying for some hidden motive.
But after reading what you said in the above quote, I'm leaning more towards believing that her friend is telling the truth. If she really asked 'what's more important, her or school' than you're dealing with a highly immature girl who is very insecure and thus believes your world should revolve around her and she cannot stand that it doesn't.
Dude, I've been there, with someone who was EXACTLY like your GF. Can I give you some advice? Run - do not walk - RUN away from this girl before you go and do something stupid...like trying to please her. Doubt that I know what I'm talking about? Well, let me tell you what will happen, and then you can decide for yourself.
Her friend has already given me the clue, she will try to blame all this on you, its all your fault she did this, she already blamed you to some extent by telling her friend 'he doesn't spend enough time with me'. Well, the loser she is cheating on you with does have time for her, because he is a loser and doesn't have any ambitions or isn't trying to make something of his life. At least, he has the time today for her, he'll get tired of her and move on to better things.
Do not make the mistake I did, which was I fell for that two bit bullsh-t line that 'you don't spend enough time with me'. So what did I do? I dropped my class load and a bunch of other stuff like totally severing all contact with my friends just for her. STUPID! Why was it stupid? Because it wasn't enough, it was never enough.
Women with these types of insecurities have them and it has NOTHING to do with you. This is ALL about her. You didn't cause them, and you cannot 'cure' them. Nothing you could do will make her feel better or more secure, because you're not the cause of her insecurity. You'll only end-up going the extra mile for her, and then some, and it will solve nothing. What it will result in is pain, anguish, resentment, and bitterness.
But, people tried to tell me the same thing and I didn't listen to them. So, you may have to learn the hard way like me. Its a tough lesson, pal, better put on your coarse exterior cuz you're going to get burned hard.