The easiest, cheapest (in the long run) fix for his problem is to have a local machinist make him a new pin from scratch with a bigger neck to take-up the slop in the reciever.
For such a small part (and given the hight price tag of the Colt), it would be money well spent.
A quick fix, would be to use a cylindrical, spring steel snap ring as a shim. Trim it so it's just big enough to circle the part of the bolt that has play in the hole.