I myself am a left handed shooter and have run into this problem many times. 20 gauge is even harder to find. I own 3 lefty shotguns, and one ambidextrous gun.
Remington 1100 g3 12 gauge (LH)
browning 425 gold sporter (LH)
Blaser f3 (LH)
Ithica featherlite 12 botttom feed and eject
There are several options as far as 20 ga in an over/under. If a pump is an option Ithaca makes the 20 in both featherlite and ultralight, but I will remind you that a pump gun kicks much harder, especially a light one. 20 ga auto's are slim to none with the exception of finding a used Remington 1187 which they used to make in LH, but fetch a high dollar because of their rarity.
Many, Many shooters who are left eye dominant have found that the ejection of a shell from a right hand chamber does not bother them. my suggestion is to have him try a 20 gauge beretta auto and see if it bothers him. They are very easy to shim with left hand shims for the stock and with almost neutral palm swell are very comfortable to shoot as a lefty. I duck hunt with a 12 gauge version of what I explained!
If you have any other questions, feel free to ask!
Jeff