I have a friend with an accounting degree who wants to transition over to the plants/refineries but all positions require or strongly encourage knowledge of SAP. How does somebody acquire that knowledge if they can't get hired into a position that uses it?

I worked for a large petrochemical company as an accountant. I started at corporate and eventually rotated to an operating site. At least at my employer, there was a focus on getting individuals to rotate from the corporate office to the plant locations. If your friend's long term goal is to work as a plant accountant, the best bet is probably looking at general accounting roles at one of the major petrochemical companies. From there he can learn SAP and work on making a move out to a plant.

Of course their other option is to keep applying and hope to find someone willing to train SAP. System knowledge is a plus but shouldn't be a total deal breaker.