You notice the person who just bought your lunch didn't tip enough

Stop being a cheap arse and buy your own lunch!

Throw a few dollars in there yourself. If something is said, just say, "Thanks for the lunch, but I'd like to at throw some in on the tip".

Throw money in. Don't make it noticeable, just slip some on the table when leaving.

I would shrug my shoulders and maybe leave a 5 while leaving

Just throw some cash on the table.

Quit being a pussy.

I would definitely try to make it right some how, esp if there was no reason to leave a bad tip. Some people just don't understand tipping and that's okay

Went to dinner a while back with my girlfriend and her parents. The server misunderstood what we meant we wanted to order our own app and drinks and ended up splitting the bill down the middle between us and them. Lucky she did though because her mom left a 3 dollar tip on an 80 dollar bill. She wasn't doing it to be rude or anything, just doesn't know any better. They come from a very small town and never eat out.

I was able to even the tip on my bill. tougher situation you are in. I would go to leave and make my way back to the table saying you left something or need to use the restroom. I've done that on occasion and always take a second to explain why the tip they got wasn't their fault, just someone not knowing better. No one gets their feelings hurt and everything is made right.

If you are dependent on tips for your living, mistakes were made. Not my fault.

I just say let me at least get the tip and I pay both of our tips.

I'll do one of two things if I notice a shitty tip. I'll pay the normal tip and keep to myself that I'm never eating with that snooty POS again, or I'll man up and confront the friend about his shitty tip. Depends on the mood.

Either way, I'm not paying extra just because my lunch partner is an a-hole.

If somebody buys my meal, they shouldn't even have to worry about the tip though.

Mind your own business and thank your friend for buying lunch. His bill, his tip.