My friend wants to buy headphones with good isolation but still decent SQ, up to $100. Honestly, I have no idea what's decent in this category and price range so any suggestions would be helpful. I'd like to get multiple options if possible. Thanks!

Recommend:
MDR V6/7506
Philips Citiscape Uptown
V-Moda M80
ATH M50
SRH440
 
The MDR 7506 is my favorite.

How about Superlux 662F, it has a nice flat response ideal for studio and it's closed, look at this review:
 
"The more i use the HD-662F the more i like them... these are the best cans i have found for tracking, the natural balance makes it easy to hear detail and the bass and guitar can stand right next to the drummer and track without complaints, i have stopped using (7506, Can't hear the bass) and k240 and HD-669 in the studio and exclusive track with HD662F and for open cans the HD-681... every musician can hear with these cans. would love to have a open version of the HD-662F it is one of the best cans if not the best i have had the privilege of listing too... natural... ( perfect balance... or as close as you can get. )
 
Doyen"
 
http://www.head-fi.org/t/531102/superlux-hd-662f-affordable-closed-ear-headphones-amazing-quality

Well if he is a bass head, go with the V-Moda Crossfade LP's, the best sounding would probably be the Shure SRH-440, the best build probably the Koss Pro DJ-100, there is also the Sony MDR7506

Gemini HSR-1000 (Takstar Pro 80) headphones, $50-$60.

Thank you guys. He's not a basshead, listens to many genres, and is looking for isolation. I think I've got some nice options now.

I liked the senn hd280 I heard at best buy

What about IEMs? Is that an option? If it is, that opens up the possibilities much more. Shure SE215, Westone UM1, etc.