When friends give me free food or drinks, I usually tip the full cost of what I was given. So if you gave me an $8 drink, I would tip you $8.
This still benefits me in that I don’t have to pay that $8 plus another tip, but I’m wondering if there’s an expectation or standard that would save me some $
Sometimes this one place gives me $20 of food before closing when I only intended to spend $5 so the upsell kinda hurts
How often does this happen? What is it costing your friends? Are they sneaking it to you? Are they the purveyor of the establishment?
In any case, if you were only going to spend $5, you don’t have to accept the full $20 worth of food/drink. You could say, “oh I was only planning on spending around $5 tonight, this is too much, but thanks.” Or if the somewhat direct approach feels awkward, max out your tip at what you were originally going to spend. If it’s not enough, they’ll stop giving you so much.