Take another look at the plugs.
I have even missed this one. The two plugs that are towards the firewall and forward on the engine tend to run exceptionally hot. This can lead to carbon tracking on the ceramic portion of the plug. This will look just like a black hairline crack of the plug. This, under load becomes an easier path to ground than jumping that gap of the plug itself resulting in a miss. Last time this happened to me (more than once) I pulled one of the plugs and found the tracking and changed in along with the wire. It was still doing it but not quite as bad. Turned out the one next to it was tracked as well. Tell your bro to have a look, it only takes a couple of minutes.
1992 Bonneville SSEi
White, grey Leather interior.
Tinted windows and blackouts on headlights.
Custom high flow exhaust, Everything replaceable - DONE
300 000 kms original engine, tranny 2 rebuilds.
Playboy Bunnies Embossed in Tint. Oh Yah!
Ever Seen a Polar Bear in a Snow Storm?