Well, keep in mind that it could be pretty huge that he's engaging in normal college activities. He might be the first person in his family to ever go to college. He might have had to overcome some serious problems (physical, mental, psychological, personal, financial, etc) in order to go to college. Basically, while his activities might seem pretty normal to you, they could seem crazy impressive to her. (I know people who are just the opposite, too. They really have no clue why I'm so impressed that they ran a mile in five minutes, because it's just something that they can do.)
Anyway, that's a bit of an aside. The problem is that someone is monopolizing the conversation and talking about something that doesn't especially interest you. I vote for giving her a few minutes to talk about it (listen actively), then changing the subject to something broader but still sort of related.
"My son joined the Ultimate Frisbee team!"
"Cool! I remember playing that when I was in college. Have you ever played it?"
"My son is rushing for a fraternity!"
"It sounds like he's really diving into the college experience. Were you ever in a sorority? I know very little about Greek life."
"My son went to class today!"
"Well, that's what he's there for, isn't it? Did you know you can take free college courses online now?"