Help! Shooting retirement dinner as a favor-- need advice.

Could you borrow a flash to mount on the D800? I think that might help. Try to get some practice with the combination beforehand if you can. Cranking the ISO will certainly help but there may be a problem with noise.

Try to use the zoom lens near its faster range if possible. I understand it may be largely beyond your control.

Good luck. The D800 is quite forgiving really, as I found out myself recently.

IainD