I'm sorry that her first reaction to a stutter was laughter. I get that it sounds funny to a lot of people, but I don't think they realize just how self-conscious it can make a person feel - and laughing usually makes it worse (especially since laughter is one of the biggest things the stuttering person was worried about, and stress can worsen it). I tend to get defensive about such things, as my cousin stuttered and I have known more than a few other people who stutter, as well. Having witnessed the embarrassment, the shyness, the hesitation, the worry over how others will react, etc., I can't bring myself to find it funny. I wouldn't go so far as to call the girlfriend a shitty person, but that reaction was certainly ignorant.