Most have limited tools and supplies.  If a customer/ Jr mechanic brings the parts and have correct or workable tools plus skills, it happens, otherwise refer to coop, lbs, next week's clinic , better mechanics, etc.

Some places have senior mechanics that check work of newbies and do it yourselfers, as brakes, steering, shifting, etc are critical, plus have someone try the bike locally for a test ride, before getting all the way home, say on a car rack and leaving brake cables loose for tire removal and kid/lady/neophyte doesn't know how to fix/adjust.

If uncomfortable with skills, don't.  Better for liability, safety, learning experience to refer, delay, learn/teach show with others.