Having had a quick read in the Elder Sign pdf rules, I think the wordings right.
FFG have given an example of JC's ability online …
… but say it's another investegators turn and they get a successful task role with JC on their adventure card, they can use his ability as an assist, only "once per turn" as stated on his card. Now, say the investigator fails the adventure card, all dice are passed to the next investigator, who can also use JC's ability, "once per turn" … and so on. So it gets used again and again during the day.
Requesting Assistance - quick rule ref: Page 11 & 12 (core Elder Sign rules)
Finally, JC would use both focus or assist during his own turn, but only one or the other at the time of the die roll if another investigator was on his adventure card!
Focusing - Quick Rule Ref: Page 10 & 11 (core Elder Sign rules)
This ability is def one of the best in the game! Hope that helps?