[stratref]

I actually had such a situation come up 2 years ago working a middle school basketball game. Away coach gets ejected and has to leave, he grabs his clipboard and is out the door before we get lined up to shoot the free throws. There are no assistants or other personnel from the school available. The home school's AD came up and asked if a parent could coach, but knowing those pesky state by-laws and having had it mentioned a few times in state clinics over the past 20 years for various sports, I explained that no, a parent could not cover unless they were a school district employee. After a few seconds of thinking she then offered herself to sit on the bench and coach the visiting team. Since she was a district employee and it was a game between schools in the same district it was fine with me. The team actually played better without their coach and ended up losing by 5 when the were down by 15 when he got tossed. The time requirement isn't explecit here though, a "reasonable" amount of time is allowed, to me that means if they are working on a solution give them time, if they are just sitting there doing nothing let them know we need someone, if they still have no ideas call it, 15 minutes to wait for a plan is too long, but if someone is driving there right now, I'll wait if necessary, as that to me is reasonable.
Something like that also happened when I was in HS. I was the official scorekeeper for the varsity and JV teams, and the JV coach gets tossed and walk to the locker room. The refferee comes over to their bench as asks if there is another coach, I jump up and grab to baseball coach who is watching and was there to supervise the varsity game, we still joke about it today 18 years later.