If he's just saying "give me $20 and I'll show you how", that sounds a bit sketchy.

If he's willing to spend some time and educate you on how hobby engines work, that might be worthwhile. THIS PAGE is a great way to learn about tuning, but sometimes you need some hands-on experience. If the engine is brand new, make sure you break in the engine properly. Specifics of how to do this will vary between posters here, but it's generally agreed that you should run about 3 tanks through the engine without even touching the throttle, followed by another 3 tanks of low-speed parking lot cruising at about 1/4 throttle. Only after that should you begin opening it up. You only get one chance to break in an engine.