Graduation or graduation........
I personally did not care one bit when I graduated HS. It was just dressing up in a gown waiting forever to hear my name in a long list of other names. College was my goal so I always focused on that. Graduating colleges was waaaay more important to me.
If the son is going to college, I personally would say the mother should be with her husband. A Ph.D. is a bigger deal than HS and the son will have more people there anyway.
I would go be with my husband the, if the son wants to, calibrate with him as a family a few days later.