Why would you take the fish to the DMV?
Isn't that the Department of Motor Vehicles?
How old is this friend? If they are a minor, I would tell their parents, and let their parents settle it...you could even offer to take the bettas at that point. If they are an "adult" I'd threaten to tell the local authorities of their plans for the fish. I'm sure they'd give them a call and advise them of exactly WHY they shouldn't partake in such activities. Harming animals is a crime.
Fighting fish is the same as Fighting dogs in the US...and it is a felony in most states, in others it is, at the least, a mis-demeanor. You might wanna tell your friend that. That'd prolly go over real well in a job interview, having to explain when that showed up on their background check.
I would not steal them..as that is a crime as well, and if you made this person mad enough..they could try to get YOU in trouble for theft. (even though bettas are kept in harm's reach at local pet shops and wal-marts...I don't think anyone would suggest you just walk out with 6 cups..you know.)
Before I did anything though..I'd make sure your friend is serious. Alot of times people tell others they are going to do things..to make themselves look "big", when in fact they are only talking a bunch of hot air.