Since the child is 13 and the graduation won't hurt him in any way, I think the choice should be his. Not participating discreetly would in no way be SS.
However, in her shoes, I would be careful of saying that finishing 8th grade isn't an achievement or that it didn't take effort. Looking back, yeah, making it to high school is not hard and expected. However, when you are in 8th grade, you have spent the last 8 years learning how to be a student, putting effort into it, etc. Telling him it wasn't much is not going to do much for his self esteem. Of course, telling him he is the specialest snowflake is not good either, so I would strive for a balance.