Hi Dan-cat, been a long time since I posted and I'm not getting on the table much these days but that is for another time.
You are not the only long time player that gets confused by this rule, it seems rather common. The rule is straightforward:
"[Section 3. Rule] 4. End of Frame, Game or Match
(a) When Black is the only object ball remaining on the table, the first score
or foul ends the frame excepting only if the following conditions both
(i) the scores are then equal; and
(ii) aggregate scores are not relevant."
and that simply means that if the difference in score is less than 7 points with only the Black on the table, the ACTUAL score no longer matters; the winner of the frame is determined by the next pot (the potter wins) or the next foul OF ANY KIND (the fouler loses).
In fact, the way to deal with this situation back when I lived in Harrogate was, in the event of a black ball frame with less than seven points difference, we would simply reset the scoreboard back to zeroes because the actual score no longer mattered at all, it was over on the next pot or foul. Likewise when the frame ends in a draw....no need to keep the scoreboard at "57 to 57" or whatever...simply go back to zeroes, respot the Black, and flip a coin for choice.
Now, with all that said, that is the proper, professional rule; a frame with your mate may require a different perspective if that is what the two of you desire. Specifically, I mean that it may depend on your interpretation of that clause number (ii) "aggregate scores are not relevant". Back in the day, there used to be the occasional odd tournament whereby each player's score was accumulated over a number of frames so each frame was played until every ball, including the last black, must be potted before racking for the next frame. Here is an example:https://www.youtube.com/watch?v=BAMEW6EASiY
Now, if you and your mate are just playing to "win the frame", then it is over the instant that he fouled on the Black. But let's say you have a small wager on the outcome of the frame, say maybe a quid a point. That is a case that perhaps can be said that aggregate points in fact DO apply. Because if you call it frame over when he fouls on the Black, then he would owe you 1 pound. But if you play on and then you pot the Black, then now he owes you 8 pounds. Or of course, playing on perhaps you will end up owing him 6 pounds. Usually, if you play a wager "per point", you must play it until every ball has been legally cleared from the table including that final Black. In fact, if you decide to concede, by the Rules, you are conceding every possible point that is still available on the table to your opponent. So let's say there are two reds left (43 points possible) and at your turn, you are 57 points down, so at a quid a point, you figure you will tuck your tail between your legs, say "That will do", and produce 57 pounds out of your wallet. In fact, you just lost an even hundred quid as you also conceded that final 43 points so you never should have conceded and instead attempted to cut your losses. Also, your concession cannot now be retracted. So you cannot feign ignorance and say, "Well, in that case, I will play on then." Pony up your hundred.
So I hope you took your one pound winnings and invested it wisely....a half-pint perhaps?
What is that worth these days?
I haven't bought one in decades.