Don't say anything. If you mention something about the $5 it could make things awkward. It could sound like "hey where's my tip?" rather than "I just wanted you to know.."
Put yourself in his shoes, someone gives you the money to go pay for something and it's less than they have given you, do you... A. Tip the person the difference with someone elses money or B. Give the person their change
How is this even debatable? If he didn't keep it and returned it: You look like a whiny douchebag if you said something to her. If he did keep it: She is unlikely going to confront him over $5. She now feels obligated to give you another 5 for two reasons: 1) You never got the tip. 2) You whined about it and you expected it. She is now out 10 and irritated at the both of you for being petty.
dont say anything, If he returned the money to her, and you whine, you could look bad. If he returned the money to her, and she said to him: "those $5 dollars were his tip, why didnt you give it to him", so maybe next time, you could receive $10 dollars next time, saying, she didnt give you a tip last time, so she is giving you tip for last time, and tip for this time.
As someone pointed out . . . . How Much do you tip . . .WITH SOMEONE ELSE'S MONEY? Do you ASSume they are a 15% tipper? or Do you take them all their money back . . . . Rocket River
so then according too you, you made a post about nothing in a thread about nothing, in a thread about nothing? insert internet meme mindblown gif
I'm surprised how many people are defending the guy who stiffed a delivery guy. Yes, it's not his money, but I would never assume that the person who is paying would want to stiff the driver. I would give her a little credit. If she then gave me a hard time about it, I would reimburse her for the tip from my own money and then spit on her.