This is a really random estimate...but from looking at prices online, they seem to be in the $200+ range. If I were in your shoes, I'd verify shipping/handling policies with the Gateway place, and make sure you get some kind of assurance on your local store, so that after you get that motherboard they don't say, "oh...uh yeah, the motherboard wasn't the problem after all."
What about Gateway? I'm guessing that notebook's no longer under warranty, huh? Anyway, a 1.6 Gig laptop isn't bad at all. If it costs less than 300 to fix, I'd go for it. Otherwise, sell the parts off and have your friend buy a new laptop. Just my $0.02.