School asked if my son can be a scribe for a classmate in the class tests

The class teacher called yesterday. A boy in my son's class finds it hard to write for long, and the school wants a classmate to write his answers while he dictates, in a separate room, on days my son isn't writing his own paper. They asked my son because his handwriting is clear and he stays calm under time pressure, apparently. He said yes before I could ask anything. I'm proud, and also a little unsure. Does this eat into his own revision time? What happens if he writes something down wrongly, who carries that? Has anyone's child done this, or had a scribe themselves? I'd like him to do it properly, without making it awkward for the other boy.