Ironically I've been planning a "daily dose of wisdom" post on this very subject, but in the meantime here's my take starting with the outside diameter (OD) of the three most common insulin syringe size needle gauges in mm:
31G .2604mm
30G .3112mm
29G .3366mm
The difference between a 31G and 29G needle is .0762mm.
For reference, a human hair averages something like .16mm, so even completely skipping the 30G size we're literally talking about a thickness difference of around half that of a human hair. I'd be willing to wager that 99.99% of people are incapable of perceiving that difference in size when it comes to how a needle "feels" and it's entirely a function of psychology and not anything determined by the various corpuscles in our skin.
Honestly, everyone starting out should begin with a 29G syringe (I still recommend 8mm over 13mm, but that's another discussion for another day), and give themselves at least a half dozen injections before considering going with a higher gauge and even then only if there are compelling reasons like excessive bleeding or other contributing factors like skin conditions such as psoriasis, scleroderma, or related complications from diabetes.
So yeah, tell your friend 29G is the way to go.