All I can think of to try is...
1. Check for a vacuum leak. Inspect *all* vacuum hoses, flex them and make sure there is no cracking. Also make sure they connect firmly to their respective ports. If you have any metal vacuum lines, make sure there are no rusted-out holes in them. Also, use caution when handling plastic t-connectors and check valves. As time goes by, they become brittle and can very easily break.
2. Check for leakage at the throttle body seat (where it bolts up to the intake itself). Unmetered air entering the engine can cause a surge in engine speeds, as air enters the engine AFTER the throttle plate. Depending on the throttle body design, you may have to pull the throttle body off the intake to check the gasket. If you do that, you might as well deep-clean any vacuum passages inside the throttle body. Make sure none of them are plugged with gunk.
3. Check for a sticking/maladjusted throttle cable. Disconnect it from the throttle lever at the throttle body, and make sure the cable moves freely within the housing. If it sticks, replace it. It might not be the cure, but it WILL become a problem and a safety issue. While the cable is disconnected from the throttle body, move the throttle linkage through it'* full range (closed to wide open). Address any issues where it'* sticking, this may require a bit of cleaning in tiny spots. Make sure the throttle cable has *SOME* give to it when it'* connected and the accelerator is at rest. I've seen people think they're doing the right thing by taking slack out of accelerator cables, and to some degree it is a good thing, but being overzealous can cause a high idle (especially for "mechanics" who are trying to "fix" something).
4. Some "mechanics" have been known to crank up the idle speed screw to "fix" a problem that exists. Someone that was under the hood might have turned the screw way out of whack to compensate for a low idle or other driveability issue. I don't typically recommend messing with this because it'* not supposed to be tweaked really. Some models actually have the idle set screw sealed for this reason.
*THIS IS SOMETHING THAT SHOULD BE ADJUSTED BY SOMEONE FAIRLY COMPETENT, BUT NOT NECESSARILY A GM GURU.* It is, however, something that should be checked as it seems your friend'* "mechanic" isn't all that reputable. And who knows what else was done under the hood over the years?