You don't need field loads for the ten. If he gets enough practice in with his o/u or another shot gun, and gets a 10 that fits him correctly he will not need much practice with the new one. (Although if he reloads the 11/4 oz load out of the heavier ten feels just like shooting a 11/8 oz load out of a 12) The great shooters around the country can pick up any gun off the shelf and shoot them well after a couple of shots. The reason being they have had enough practice and experience to follow the basic rules of shooting. Keeping the head down, the proper lead, follow through, range of the target, capibility of the guage and load, etc. So if to answer his question there is no reason for him not to get a 10. The ten can offer him denser patterns and longer range if he has the ability, or improves his ability to shoot. And if I recall correctly he is looking for a waterfowl gun not an all around gun.
As for the 410 killing as may birds as any other gun, this is true in the hands of a shooter who knows how to correctly use one. I know plenty of people who use the smaller gauges for the challenge. But they do have the ability and experience to effectively kill birds. They know the range and capability of themselves and the gun. In fact I'll bet you in there hands, on average, they kill a heck of a lot more ducks than you average pass shooter!
I can also tell you (around here) most people reload first, to save cost. Second to build a better load. Of course this is in my neck of the woods, which is not the wealthiest around.
I say, don't listen to others because I believe you can make a better choice for yourself if you research and find the information you need to get what you want. People are to bias to listen to in these matters. Proven facts and your own feeling are better to go off of, not some one else's option.
Of course as ironic as it is this is still is only just my opinion. :toofunny: