Well I admit I can only stomach my children's bodily fluids - if someone else threw up near me, I wouldn't hang around any longer than necessary! But I wouldn't be snotty and judgemental about it either, and would offer the parent sympathy and tissues or wet wipes if I had them.
At the very least, I would just leave the area without any comment, but I've been in that position too many times as a mom to just walk away without trying to help.
What the people in our situation said was "How gross - you know there IS a bathroom right over there" accompanied with the various "ewww, yuck" sounds and faces.
Fortunately, the manager of the restaurant was very nice - since the bathroom was STILL occupied when we came back in, he gave my son a wet towel to wash his face and a glass of water.
They really ought to have more than one bathroom though.