30 stone = 190 kg
Since he is known to be over 30 stone, he's probably 200kg+
all in just one corner of your passenger compartment, (which iirc your vehicle manual says can take 500kg total there?)
Discussed this yesterday at work over lunch - we're scientists and engineers;
I've a STEM PhD, with 30 years development work mostly in the automotive and aerospace sectors
100 years total experience between us of working for automative OEMs and Tier 1 suppliers, developing new vehicles and automotive tech
My opinion, endorsed by colleagues:
We can't say what would happen to your car when you use it for a purpose it wasn't designed to withstand
and no mass market vehicle is specifically designed for a 200 kg passenger
Vehicles are designed - and tested - for a range of standard occupant sizes, but the maximum size is the "95th percentile male," weighing 100kg
Your neighbour needs specially adapted / designed transport, so that he and the other occupants can travel in safety and comfort.
Possible areas of concern are: seat, seat fixings, suspension, safety systems;
also any handgrip he uses to get into the car might tear away from its fittings
The suspension might be over-stressed by the unbalanced weight when you turn corners, hit a pothole etc or brake sharply etc
In the case of sharp braking / accident - concern for: seatbelt anchorages & restraint system, seat, head support, airbags.
In fact, since his size would bring him too close to any airbags, he would likely be struck by them before they have inflated fully and possibly injured from that.
The restraints system might not hold him properly in place, as they are designed to restrain much lighter people
A massive modern SUV wouldn't be developed for a 200kg passenger either, but it would inherently have much more safety margin for high loads
Your car is old and medium-sized
Also, once a vehicle is several years old, it might have developed tiny defects that don't matter in normal use,
but under extreme loading - which travelling with a 200 kg oassenger is - could cause those components, if stressed, to be noticeably damaged
You might get away with it once or 20 times, or you might see damage after the first trip
Personally, I'd avoid doing such a risky favour, which could go wrong for you both.