I hope it all goes well. I know I found it really difficult when my daughter had her op, her jaw did affect her aesthetically but also made eating difficult, caused jaw pain and headaches and she is so happy that was all resolved.
All you can do is be there for her and offer support. If you want some practical things you can do I can suggest:
food - my daughter was offered mashed banana for every meal after her op. Sometimes she got icecream or jelly but have you ever looked at mashed banana, not very appetising. My DD did lose weight after the op but I did my best. They let me use the ward kitchen so worth asking about that. I then made some easy to swallow things she liked.
Help with her mouth hygiene, they just don't have time on the ward. They will probably give her some sponges on sticks, look like pink lollipops. Clean round her mouth with these and she might like to suck water from them as well.
Encourage her to take her pain killers, my daughter had stuff like Calpol as she couldn't swallow the tablets at first. Tastes horrible but don't let the pain get out of hand.
Encourage lots of visits, I found my daughter could get low in the first few days and visitors brightened her up.
Dont be shocked, if it is anything like my DDs op she will be very swollen and bruised. Don't be tempted to say she looks great, more of a problem for my DD as her appearance was part of the reason she had the op. We all tried to tell her she didn't look bad but it made her panic as she got convinced that was how she was going to look. I learned to say, "The bruising is bad now but it is getting better." Sometimes it is hard to get it right.
The worst is over in a few days but I honestly felt it was months before all the swelling went and she was completely over it. I know a couple of people who have very similar jaw surgery, some took a bit longer to get on their feet but as I saw in previous post my DD went to an 18th birthday party within a week of her op. She wasn't feeling great, she wasn't looking great but she wasn't prepared to miss the fun.
Is the op soon? Just try to focus on the benefits, I am sure you will be a great help to her.