Install something else and file for the rebate its way cheaper in the long run.
I think there is a code that says the boiler must be so many feet away from the service entrance but I could be all wrong about that. I know when I switched from oil to gas I spent about $1500 in parts and had I gone with a lower efficiency direct vent gas boiler it would have been over $2k and thats not including the boiler.
I don't know the specifics of the job but I would say that $6-7K for a new gas boiler installed is about right. i would however try to convince your friend to go with a condensing boiler for the little bit of extra up front cost