Is it legal to AirDrop an app to another MacBook?

what does the little person inside your head say when you ask the question?

That completely depends on the license agreement for the software. Some software says you get one installation. Some says two. Some says up to 5 in a household. But most that allow multiple installations still say they are for only one user, and it sounds like your friend would count as user #2. You have to read the license agreement for that software to figure out if you're breaking the law.