My friend's boyfriend says she beats him. When I ask her, she says she gives him a friendly thump or hit on the arm, but never starts punching him in the face or kicking him or anything, she says she's always done it and it's how she's been raised.

What should I do? That sure doesn't sound like a beating to me!

Well, obviously one party is exaggerating, you just need to find out who it is. Maybe your friend's boyfriend was just joking? Exaggeration for the sake of humour? Confront him about it again. If you tell him that it's been bugging you, and that you're beginning to see your friend in a different perspective upon hearing about this, he'll be more inclined to be honest with you. Make sure to keep a serious facade, but don't act like you're about to do anything irrational. If he senses that you plan to get involved, he might get scared and lie to you / not disclose important details.
How close are you with your friend? Do you personally consider her a violent person? If they've been raised differently, they need to recognise this. What he views as a punch, she might consider a light tap. Tell your friend that she needs to talk to her boyfriend about this. A relationship can't function if there's going to be confusion about something like this. Observe how they act in public. Are they disbonded in any way? Does he seem uncomfortable around her?

Probably just exaggerating.

(Big controversial bastard that I am though, I have this sudden urge for someone to post a similar thread about a girl getting beaten by her guy, just so I can give the same response and get brutally e-murdered for it).

Well surely, if you want to do anything about it, you have to find out who is telling the truth? she is either 'beating' him, or giving him a 'friendly thump'? but you kinda need to know which.

I'm going to give my mum a friendly thump tonight. A couple of days ago I gave her a friendly slap with some ten pound notes, but she wasn't pleased.