Go ahead and book for everyone. If your friend is unable to make it, there will not be a penalty, they will just seat the group that does show up.
The only time you really have to worry is when the meal is prepaid, like CRT, or it has a CreditCard hold, like the place in Norway. Then you will be responsible for the entire party.
Actually, you might not be at the places with the CC hold, if part of the group shows up, but Disney has the right to charge you a $10 "no show" fee there.
During our annual trips to WDW we always make ADRs for our entire group for every meal. There is usually one or two that hubby doesn't make due to a golf game running late or I might not make due to a tired kiddo. It has never been a problem.
Have a great trip!