I'm not a 1911 expert, but I think it can be difficult to judge the value of a gun that old, because so much depends on the precise condition. For some people "a little bit of surface rust" might be a deal-breaker, for others it may just be part of the charm. Pictures would probably help.
I would guess that it would certainly be worth at least $1500, and maybe a lot more - maybe up to 3 or 4 thousand. An original early Colt like that in good condition could be very valuable, I'd imagine.
Have you tried surfing around Gunsamerica.com, just to see if there are other similar listings?
Is he looking to sell it, or are you just curious about value (for insurance purposes, etc.)? Personally, I would love to have a WW1-era Colt 1911, but probably cannot afford one like that.