Addendum...
An alternative to granting Chris full access to your sexually-hungry woman friend (occasionally) would be based on your mention, a while back, that you were considering the desirability of keeping him in a male chastity device. If you are doing this, or are still thinking it may be a good way to go, you could invite your woman friend over from time to time and require Chris to service her using only his tongue and hands, while requiring him to continue wearing his chastity device, under your supervision (i.e., while you watch... or not, depending on your preference).
Is your woman friend aware of the nature of your relationship with Chris? I.e., that you are the boss, he is your submissive BF (cuckold-in-waiting, as it were), you can fuck whomever you wish but he must remain faithful to you... up 'til now, anyway... and, in an overall sense, he does what you tell him to do?
—Custer