Well the law in question is that it's a copyright violation to give a public performance without a license to do so. I haven't ever studied copyright law, but in the areas of law I have studied "public" usually means that basically any random person is invited to come participate, though charging a cover doesn't make something not-public, and neither does an age limit, etc. 17 USC 1 is the US copyright code, and section 101 defines a public performance as
So watching a laptop on the grass in front of a dorm does fall under the literal meaning of the statute. However, I can't imagine any federal court would ever enforce it that way. The obvious intent of that definition is to prevent someone from showing a copyrighted work to a large number of people, and watching a movie on your own laptop does not implicate that purpose. The number of people who can watch a laptop at once is very limited, and if people started gathering to watch I'm sure your friend would have gotten very uncomfortable very quickly and stopped the movie or gone inside or something.
So, having done a little more research, my conclusion is that one of two things happened:
1) The cop got overly literal with the term "public performance", not based on anything other than his own reading of the rule, or
2) The cop was bored and felt like being a dick. Possibly both.
Also, a good rule of thumb is to take anything a cop says about what the law is with a MASSIVE grain of salt. They have an immense amount of practical experience, but almost no formal training about what the laws mean.
Also, customary disclaimer: none of what I've said here is legal advice, it's all just for general discussion purposes.
*Edit* - Wow, I wrote a lot more than I intended to when I started.