It is not uncommon to see people struggle with guns that are too big for their hand. Most typically if the trigger finger drags on the frame as they actuate the trigger, they'll struggle. At work, we have to carry Sigs but have a range of options as far as model. I've had fairly good luck improving someeones shooting by getting them in a smaller gripped weapon. A lot of people don't like single stack guns but if they demonstrably shoot them better, I encourage them to make the switch and practice reloading more.
I used to teach a lot of ccw classes and saw everything from Hi Points to even a Ruger Super Blackhawk used by students. I had a couple of Model 10 revolvers and G19 pistols available as loaner guns if needed, but I never tried to force a student to change guns. If their gun was malfunctioning or if they simply could not manipulate it properly then I would have them try another gun. Part of the classroom lecture covered choosing a gun, and I had many students ask to try one of the loaner guns to see the difference compared to what they brought. They smart ones didn't need anyone to tell them which gun was better for them.
Ken, I hate to say it, but your instructor is correct. You do need to get rid of that P30. I know your gonna need a new ride, and im all about helping a fellow shooter out. Send me your address and I'll have $300 bucks heading your way for that old, poor shooting pistol. No need to thank me, just pass it on.
When teaching CHP classes, I have recommended to the few that came in with High Points that, if they have an opportunity to sell it for close to what they spent on it, replacing it with a police trade-in Glock will be a much more dependable option.
Aside from that, the only time I've suggested a different gun (and hated saying it) was the older left-handed gentleman that had just bought a .40 Sig. With all of the issues he was having operating the controls (I wanted to smack the guy downstairs that talked him into it), I listed off a few brands that would be much more accomodating of a leftie like himself.
I wish more gun store sales people understood that, just because they like a particular model, doesn't make it the perfect model for absolutely everyone.
As noted, the OP's instructor was an idiot.
I tell folks all the time that they would be better off with a different firearm. For example:
-- Officers with gen 3 G22's that cannot make it though a magazine without a malfunction...I recommend they consider a gen 3 G17.
-- SWAT team members with crap 1911's like Kimber's that cannot seem to finish a single drill without multiple malfunctions...I recommend they consider either a complete overhaul by a solid pistol-smith, get a different .45 ACP pistol (ie. SA 1911 Pro, M&P45, HK45, or even a G21sf), or better yet just use their normal duty handgun.
-- Left handed officers with large hands who frequently inadvertently dump the mag from their Sig P series pistols I frequently suggest they consider trying a different duty pistol like an M&P, Glock, or HK.
-- Officers using .25, .32, .380 for off-duty use; yup, I recommend they consider using a different pistol like a G19, M&P, HK P30 or HK45c, etc...