So it was a beginner who could not bodydrag yet ?
And a LONG way apart from the board ?
You could surf his/her board downwind, and then bodydrag to your board yourself.
I would do that - especially because you are skilled at upwind bodydragging, AND, because a hydrofoilboard travels extremely fast downwind - faster than any other board thats for sure, so you could also just wait for the board.
The foil makes a pumping motion just like a dolphin, so when the water is not completely flat (and it can not be if a TT is out), even the smallest non breaking waves drives the board downwind fast.
I have often experienced in marginal wind, to drop my kite in the water during a trick attempt or falling, and get away from my board.
Then I drag myself ashore by means of the kite, so I can relaunch the kite when it hits the shore.
But often, the board has travelled so fast, that it almost comes ashore first (not quite, but close)
We also know it when positioning the board for a strapless start, where it points dead downwind while you position yourself and the kite, and then if we dont have a firm grip on the board tail, it will "ride" several meters away from you, so you have to bodydrag with the kite, to catch up or it will ride away from you
Havent tried picking up boards when on a foilboard, so can not comment if a smarter way...
But it is the safest way IMO, to ride the lost board downwind, and then wait and/or bodydrag to your hydrofoil
Saving somebodys kite ?
THAT I would never do if windy, on any board - have seen too many "close calls" where it almost ended deadly because of linetangles and kites suddenly powering up, so they got caught in the lines
In light wind though, it is a different situation and not risky the same way