Even without an instructor it's pretty difficult! The PA28 is the most docile aircraft that I've flown, more so than the PA38 which flips into a spin quite easily by comparison hence "traumahawk".
I rent aircraft from many different schools, mainly in the USA and always with a pax. and often with a different passenger and I always take them with me on the rental check out ride. I always provide a sick bag as insurance against being “pebledashed”.
The USA check ride includes full power on and power off stalls and steep turns and at least that way, both you and the passenger gets to learn if there are likely to be any problems during the rest of the flying. A check ride is pretty much as bad as it’s going to get for them and if they are sick, the instructor can take over the control whilst you pacify the casualty (hasn’t happened yet).
A couple of hours (total of 4 hours for two pilots) should be all that you need for “differences training” providing that you are “current”. And that would be for switching to a high wing too.
So far as the other mob are concerned, just let it go - you wont win! (I like the pig analogy)
Complain with your feet – providing you haven’t paid up front – don’t ever pay up front!