Uh, lets just remember that these devices, especially the electric types, FAIL!
I, by no means, am trying to sieze some higher ground here, but no one's mentioned the importance of curing "your friend's" problem with navigation. I picture myself night diving in a current relying solely on a strobe flashing in the murk to get me back to my boat. If the light goes out and I miss the anchor line on the way back downcurrent...
I'm just throwing it out here but in the above situation I've used a reel line tied to the anchor line or dived perpendicular to my anchor line to help insure my survival.
And I've rescued people hopelessly downcurrent from their anchored boat.
don