How is his sleep and eating right now?

My daughter (2yrs) is a very bright, happy and 'good' kid but if she is lacking in sleep (even just a bit), has missed a meal or is feeling unwell she has ZERO coping skills and is meltdown central...including hitting. It is NOT that she is undisciplined or a terror. She just completely loses any ability to control herself.

The best thing I can do is stay on top of sleep and food. When I know those are lacking then I need to remind myself to be a tad more gentle and forgiving. Obviously it isn't a free pass to be hitting or flipping out...but generally I am rather firm when it comes to such behaviors and if I know she is overtired or hungry then I will be a bit more forgiving and try to give extra hugs to help her cope until I can get her rested with a full belly.

If this is the case for your DCB then being firm should help get him back on track and when he is feeling 100% the hitting will quite possible stop.

As for the parents...just give them a gentle debriefing. He is hitting again, you are tackling it in this manner, you hope that with a good nights rest he is back on track. I wouldn't get too detailed after one day...it is quite likely related to him having been unwell and tomorrow may be much smoother in which case you did a big spiel for no reason. Make note of it, if the behaviors continue then have the parents crack down again!