How soon before a flight can a 1K cancel a reward flight before scheduled departure

We're scheduled to do LAX/HNL this coming Sunday with friends, all on award tickets. My friend's dad has been really ill and he's been taking care of him. He thinks his brother can take over for him so that he can go with us, but there's the possiblity that his brother won't be able to make it, hence our friend not being able to make the trip.

I know that 1K's can cancel or change award flights without penalties, but is there any time limit on when you can cancel an award? Also, Can I cancel just one of three seats on a PNR?

Thanks for the help.