Helping a friend with a new car. He will be running 10” wide wheels. Knows he can fit a 29x9 slick but a 29x10 is going be tight and might rub.
Is he really going to be giving up a lot if he just stays with the 29x9?

no

So the 9” will hook as good as the 10”

yes, have YOU ever watched an NHRA stock eliminator car launch? It's more about the suspension than the tire. If your friend's car is setup correctly it will have no issue hooking, if it's not, it won't hook on a 10" tire either.

I used to run an index class that required a 9" tire and I was skeptical and shocked when the car launched exactly the same as it did with the 14's I took off, but that was on a well prepped track. The 9's weren't nearly as good when the track went marginal.
Consider the track prep where the car will be run. Will it run on a 110 degree plus starting line.
Consider the vertical contact patch of the 1" wider tire. Your 1" might become 5" depending on your application.
Consider the wheel width and it's affect on the section width of the 10" tire. That's why they list the width of the wheel they used to give tire measurements in the tire specs. The section width is what rubs.

I once took off 28X10s and put 28X9s on a 10 inch rim, when I couldn't get 28X10s. Looked goofy but car hooked just fine.JUst had to run more air pressure. Like Ed says though that car flat out has always worked.