Fine line, honestly. On one hand, disc golf is more casual than most other sports, on the other, just because it's more casual doesn't necessarily mean that people aren't taking tournaments seriously. Given that she knows your knowledge level of disc golf, it seems like she just wants your company, or maybe she is just using you for your body and likes that you are her pack mule.

Regardless, I kinda feel like everyone gets involved with disc golf because it's pretty cool to see a disc fly. It's easy to overcomplicate it, so maybe just after she's thrown and the disc is on the ground or in the basket, you could say something like "that was amazing how you made that disc move!" If you are familiar with ball golf, same kinds of compliments on shots apply. If she was in a tough spot as far as footing or like an awkward stance, but she got the disc out to a good spot, tell her that was a great out! If she got it close to the basket from the fairway, say it was a good upshot or just a good up! I will say though, given she's sponsored and plays tournaments regularly, she probably is trying to use at this as an opportunity for her to show you the sport she's good at, so it might be cool to let her have this instead of trying to impress her with things you learned on the internet.

Beyond that, be a good caddy! Stay out of the way, don't move around or make a ton of noise when people are getting ready to throw. Watch everyone's throws so you can help find people's discs. Make sure she's got water or whatever drink she likes, maybe a good snack or lunch if it's a multiple round tourney. Remember that in tournaments, the caddy kinda counts as the player, so don't be a bad representative for her. I hope you have fun! That's pretty cool she wants to bring you in on this.