The bottom line is, if this is on the level, if you can verify his claims, if you'll get a legitimate invoice, your warranty on the machine itself should not be violated. Apple simply won't cover the SSD itself, since they didn't sell it to you.
I work at an Apple Specialist (basically the equivalent of a privately-owned Apple Store). I've dealt with a lot of certified technicians both as a customer, and working alongside them. I can tell you, most of them, as long as you're nice, and they're not having some kind of horrible day, are willing to work with you as far as your AppleCare warranty is concerned.
They're not going to stiff you because they see an aftermarket SSD is installed. They're not insurance adjusters, trained to scrupulously screw you out of your money. If you bring in your machine for some work and it doesn't demonstrate obvious negligence (which it shouldn't, if this is on the level), your warranty on the machine and the rest of its components should be fine.
If, however, this guy *isn't* on the level, it could pose problems. At my job, we receive machines occasionally with aftermarket RAM, for example, installed improperly. Customers (or "their techy friend") have jammed it in the DIMMs, broken some pins, and act like nothing is wrong.
In summary:
Find out about the questions I mentioned before.
Check with Apple to see if this is, in fact, an Apple Certified Technician, working at an ASP.
http://www.apple.com/buy/locator/service/
If it is, go for it. Why pay gobs of money when you don't have to?