Quoted:
So my girlfriend asked me if I would find a date for her way less attractive friend that is some what over weight. Well she named off two of my really good friends I almost feel like I'm slapping them in the face to ask them to take her with us.
Would you ask them, or tell her that they said no thanks, and leave it up to her to find her friend a date?
Do your gentlemanly duty sir. Acquire a pic of the friend in question, and show it to your two friends, while asking them if they would go on a date with her. Don't goad them or guilt them with anything, just say "Hey [gf's name] was wondering if you would be willing to take her friend on a date with us." Show them the picture. If they say no, thank them and then come back to the girlfriend and say "Sorry, but they weren't interested."
That way you aren't being a dick and lying to her, but you're also not being a dick to your friends by making them miserable.