It would seem like a good idea to immediately get over to the lav if you were vomiting.

This sounds like a case of the stomach flu.

The F/A might have suggested that the pax use the lav, but the decision rested with the passenger and his family.

I don't know if there are official guidelines for this type of situation.

If your flight had empty seats elsewhere on the a/c I would have moved to another seat.

Did anyone move, or ask the FA for permission to change seats?

The FA could have possibly offered the pax additional motion sickness bags, but what else were they suppose to do?

RC