Nice thought on your part. Kudos to In-N-Out if that is their regular policy.
Reminds me of a time in the local grocery when a mother with a cart of food was short a couple of bucks for the final tab, so I told the cashier to close out her tab and put the rest of the stuff on my bill. The woman seemed embarrassed but thanked me for helping out.
As she was leaving I overheard her ask the Assistant Manager why I would do that for her. The AM said, "he's just a nice guy, one of our regulars".
Made her grateful for small favors and me feel like a million dollars.