I strongly agree!! I have bought lunch for contractors whom I AM PAYING for a job. The house is finished now, but I'll still offer the cable guy, or electrician, or UPS driver etc. a bottled water or Gatorade when they arrive, and send them away with another for the (Texas) hot drive to the next job.
To the OP, the most charitable description of your "friends" is "clueless and inconsiderate. The more appropriate description is "bloodsucking users". They should have been crushing you with kindness for the favor you did them. They already seem to have an outsized sense of entitlement. Imagine what they'll be like if they have paid you and feel you "owe them" the work.
I do car repairs for my in-laws (occasionally pretty extensive jobs) and they always try to pay me. I never accept, but they'll immediately set a date for a nice steak or barbecue dinner to show appreciation. Plus, my father-in-law frequents garage/estate sales and occasionally brings me a great tool or other find and won't let me even pay his cost. It's a pleasure to do favors for people like this.
RUN, do not walk, from this potentially disastous "job". These are not nice people.