I don't mean to sound like a skeptic, but....
If he's a "certified" mechanic, why is he working landscaping and what areas is he certified in.
Someone may sound like they know what they're talking about, but be skeptical that he knows how to do things right. I had bad experience with "mechanic" friends before i started working on cars myself.
If he does know what he's talking and can actually do things right and he just happens to be a ricer that should be ok, but just make sure he doesn't slap on any stickers on your car while you're not looking or add a 10ft tall wing
------------------
Mustang Parts Specialties ripped me off, it could happen to you as well.
'84 Mustang 5.0 LX
my LX
'89 Mustang GT
my GT