firstly don't lie and under perform the car, if he can't handle it, that's his call.
if he wants as much as you can get out of it, then if you do the mapping then you should do that within safety margins.
it's not your concern how he insures it or drives it.
however if you're driving down the road, do you want to meet this lad coming the other way?
if not, then choose not to do the mapping.
no doubt he'll find someone and they'll do the work, always someone willing to do the work.

you could always have a chat with this lad and if he gives the right answers do some work, but agree how far you're prepared to go with it.

at the end of the day you are liable for damage during mapping and that's about it.