I agree, if you seriously worry about a poor social life for 3 months or your gf can't understand the opportunity the situation presents, its either time to get more dedication or a new gf.
I admire your friend's dedication to his gf and you being a great friend in understanding. However, you have to consider this: you're just starting out now..once your startup is in its later phases, you will be MUCH busier. If the gf can't understand now, would she understand later?
Yeah, the social stuff and the girlfriend are not really top issues, but still things we thought about. In fairness, my co-founder told me flat out that the gf thing was not a huge issue for him, simply because, like you said, it's going to be rough for them either way, and she understands we need to make this work.
As for the social aspect, I'll admit it - I'd like to be able to occasionally have some fun outside work. Do I think we're going to be partying every night (either in Cambridge or Denver)? Hell, no. Would I like to have some good friends around? Well, yeah. Is not having a group of friends available a con? Yeah, it is. My life is both work and play. I have no illusions about the ratio of those two while creating a startup, but I'd still like my limited play time to be of high quality.
Plus, It's very unlikely it would be for only three months - if it were just that, I'd wouldn't worry about it. But the reality is that if we move to Cambridge, I we wouldn't be leaving unless we had a good reason.