The very first thing I would suggest would be to start with the gun he hunts with.
The idea of sporting clays was offseason practice and entertainment. I also think the chief benefit to the novice clays shooter is learning to guage yardage and application of correct choke. If his gun is fixed choke and in Modified or Improved Cylinder, fine also. I have watched guys just starting out
with a new gun make themselves crazy "choke swappin" stand to stand. If they have a gun threaded for choke tubes leave him with IC , MOD tubes and hide the rest. If he hunts with a semi auto same thing. But someone wanting to switch to a semi auto. I would first suggest an action thats recoil operated like the Browning A500, or any of the Benelli, Franchi , maybe not the Stoeger same internals but I havent used one. If he insists on a gas auto I would set him down and have him completely breakdown the action 3 times like you would for cleaning. Then ask him if thinks cleaning would be a pain, it is, and eventually he wont clean it like he's supposed to and he'll have problems shooting crap shells, even good shells will foul it up.
My opinion a pump action is actually an advantage, since it operates on arm power, since you'll so be buying shells bulk, and soon looking for a bargain,you wont buy 3 cases of some thing, and find out it wont operate "your " gun. Only advantages to O/U is choice of 2 chokes, internally not effected by powder fouling. still some ammo sensitivity, a nice one, with backbored barrels and porting, will leave you in search of a shell that Kicks just enough to reset the action so you can use both barrels.The cheaper guns dont do this. If you have to have a ported backbored gun Mossberg 835 is least expensive, good quality barrel on the market. Heavy enough to swing nice, and help with recoil. 0 ammo sesitivity, and I hear you can hunt with them, so all that good practice won't go to waste. Pumps properly operated are an advantage over an auto. Especially if your one of these guys that cleans your gun once a year wether it needs it or not. I also would recommend Remington 870. Express fine also. And for every 40 guns with problems theres 40,000 puttering along just fine.
I also think basing your choice on what the expert down at the store says, is a mistake. First what he likes is not what you like. Second asking the merchant is like asking people that work in an emergency room what they think about guns. THEY ONLY SEE THE PROBLEMS. And they just might have a motive to sell guns. But thats just my opinion