I'm not a professional, but I help people out from time to time. Usually, it's with diagnostics, then I figure out if I can do it in an afternoon while drinking their beer and using them as a helper. If it's more involved, I always refer them to a shop. People tend to appreciate it and expect nothing more.
I don’t expect to get paid to work on a car I volunteer to work on but then people get offended or question the price of parts. Or don’t offer to replace the consumables I may use off my shelf. That irritates me.
There are exceptions to all of this. There are friends and family I will drop everything at a moments notice to drive many hours day or night to help. But that list is small.
I believe in Karma of sorts. Helping friends and family, can come back to you in many ways. But...A work mate seems to be just that, a work mate.