Upon rereading the beholder's entry, I noticed that it can't deliberately target a magic item with its disintegration ray - only nonmagical objects and "creations of magical force". To me, that means that magic items that a creature is wearing or carrying when their body is disintegrated do not get destroyed. That seems to be in keeping with both the disintegrate spell and also 5e in general. It's incredibly hard to damage a PC's stuff in this edition. You can even light a PC on fire and their clothes won't necessarily burn away.
Anyway, here's what I've told my player:
The PC's body is now "a pile of fine gray dust". They'll need either true resurrection (good luck!) or wish (maybe) to return to life as they are. Alternatively, I am willing to count the dust as "a piece of a dead humanoid" for the purposes of reincarnate, since it comes with its own drawbacks (even if you get lucky enough to roll your original race, it'll still be a different body).
As for the magic items, I considered giving each one a save vs being destroyed but decided not to, since it seems like the intent is that they would not be disintegrated with the PC's body since a beholder wouldn't be able to deliberately disintegrate them even if they were just lying on the ground unattended.
The other PCs will gather up the magic items (and presumably the dust) and then the player will make a new PC to play until such time as the others are able to bring the disintegrated PC back to life (either in her own body or a different one). He may even decide to keep playing the new PC and not bother with the dead one, for all I know ...