If you do the research, so you know what you are looking at, I'd look for a bag on Craigslist or eBay. I like Craigslist better because prices tend to be lower and you aren't bidding against someone who just has to have whatever you are bidding on.

A down bag for $100? Don't know about that. Look through the Sponsor ads on the site here which have a lot of bags listed.

A reasonable choice for a casual user like your friend, in my opinion, would be something like a North Face Cats Meow. I have an old one. Not $60 new, but used ones should be fairly common since it is a popular bag.
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