Had something similar to this happen to me on my last musical. Not quite the same, but similar. We had a faculty member who played one of the minor roles who didn't show up to a mationee, the role wasn't much, but he had a song (discovered later that he had to deal with an aggressive student), the booth was warned not too well in advance, but we soon discovered that the stage left ASM had a crew that could run set changes without him for a few moments, luckily had a costume, and knew all the lines and choreography.
Assuming that this is not the case, and that the only option is to use this "guy who someone knows", I would attack this in a few ways.
First, I would see if we have any wireless earpieces- bluetooth, whatever. At this point nearly anything would work- and connect one to an ASM or assistant with a script and his blocking. He needs to know where to go, and assuming that "Benjamin" is like the actor I'm assuming he is, we haven't seen his personal script since the first read. Problems are likely with the earpiece, yes, but it is just the blocking. Not that blocking isn't important, but he has a script. If something goes wrong hopefully he can improvise effectively.
Second option being to write the blocking into the script the "guy" will be using- but that means extra lag for him to read and comprehend it.
Next, I would send someone I could trust- an ASM with limited responsibilities perhaps, or a trusted member of the production if I couldn't do it myself- to sit down with "Guy" and run the entire show, all the lines, blocking, and general bru-ha-ha with him. It's only two o'clock. With two people running the show by themselves, it should take maybe two hours, tops. That gives us four hours to curtains.
If we don't have a costume, he isn't going to do us as much good. Schedule a costume fitting for four thirty if possible, or perhaps at the same time as his "read-through" if impossible to do otherwise. Hopefully this could take less than an hour (we are speaking hypothetically, no?) Leaving us with two and a half hours to curtain, and approximately half an hour before other actors begin showing up.
Thirty minutes to show him props and explain their function to the show. Get the guy enough information to improvise if he has to.
Six o' clock, introduce him to the cast, explain his position. Hand him over to a responsible party to prepare him to go on, then go get your things ready.
Cross fingers. Remind self to kill Benjamin. Offer sacrifice of caffeine to the theatre gods. Hope for the best.
