Broadband Internet & ISP Recommendations?

Is there a way to empirically prove who has been leaching off a wifi connection?

My dad has been complaining of high bills in recent months from BT (no surprises there) but they claim he's been using 90+GB a month when all he does is check emails and occasionally watch fly-tying videos.

He lives in a small semi detached house with 5 or 6 other houses within 50 feet of him (next door, next door + 1, across the road etc).

1. Is there a way to trace the culprit?
   
2. If so, what can be done (apart from a baseball bat to every electronic device they own).
   
3. If not, is there a way to claim back the $$$s this has cost him over the past 6 months? He spoke to a call center this morning for over an hour and being deaf as a post struggled to understand what the fuck she was going on about but he's under the impression they're not prepared to refund the excess.
   
   

I'm assuming someone has cracked the password on the router? What else could be possible? Someone accessed his computer on the network and read wifi password from there? He uses a mac and has basically zero competence with anything more technically demanding than an abacus so I suspect there's nothing in the way of security software installed which won't help his chances.