What kind of "collection" does he have? That makes a difference.
If the collection is an "accumulation" of relatively recent production commercial guns, that are all either still available or are more valued for their utility then their scarcity or collectability, I'd go with putting ads in the paper or putting them on consignment at a local shop. I'm talking about modern hunting rifles, recent production pistols and things like that.
If the collection includes recently imported, common, mil-surps (Mosin Nagants, Yugo Mausers, SKS's, etc), I'd probably still go with ads in the paper and putting them on consignement.
If the collection includes good quality U.S. military surplus rifles and pistols, I'd contact Scott Duff (do a web search). He could list the good quality Garands, Carbines, 1903 Springfields, military 1911's, etc, on his website. I believe he buys outright and resells, but he may do consignment. He only does collectable military stuff, with an emphasis on U.S. arms. The nicer the item, the more he would be interested.
Now, if he was a true "Collector" of something that there is a decent amount of collector interest in (but aren't USGI stuff), and where the prices per firearm typically are at least the $1,000 apiece mark, then I'd start looking at speciality auction houses. Collectible shotguns, high-end sporting rifles, etc.
If the collection includes Colt SAA's or collectible S&W revolvers, I'd contact Jim Supica at Old Town Station. He puts out a catalog several times a year of firearms and buys whole collections. He's also done mil-surps and commercial guns, but specializes in the older Colt and S&W guns.
If he has internet access and is computer savy, he could put them up for sale on the various gun boards. For mil-surp stuff,
www.gunboards.com is a great choice. THR would be a good place for defensive pistols and assault rifles, etc.
Hope this helps.