Have him check with HR on this one; AFAIK, he may be able to transfer to Denver from the West Coast and only loose center and local union(feeder list) seniority. Haven't looked in a while, but I know drivers can transfer from So.Cal. as far north as Alaska, west to Hawaii, and east to at least New Mexico(maybe further) I can't remember if Texas or Colorado is on the list or not. With 15 years company seniority he should be high on the transfer list and shouldn't have to wait too long depending on the location transferring to. The transfer list goes up in November(I believe). I transfered 11 years ago from local 396 to local 542 with only 8 years company seniority in only10 months after bidding to transfer to a "retirement" center. I suggest doing some research into some of the hubs and centers before selecting one, unless he is not picky about type of routes available(resi, rural, freight and etc.). I myself am partial to the resi/rural routes that are loaded by preloaders at the hub.
Hope this helps...and good luck!