Do you have a copy of the picture?
It could be the liquid/water marker, a little (usually white) sticker that will turn pink/red when exposed to liquid,
AFAIK there's one on the side of the battery tray just below the SIM card on my everyday Bold, there may be more internally although I had a Bold in peices yesterday (water damaged as it happens.) and didn't see any, they do rust nicely inside though.
If your friend is absolutely certain the phone has not been exposed to liquid it might be worth argueing the toss with 02 as phone companies have been known to cite water damage in cases where it hasn't been. Although I'd imagine they'd stick to their guns here.
If worst comes to worst and they're saying the waranty is void as it is, the trackball itself is fairly simple and cheap to replace, I've no idea if that'll solve the problem but I guess they've nothing to lose by trying.