Get a fly mask that covers her nose, it'll apply some shade and some fly masks protect against UV rays. How long do you keep her out? Try finding or creating a shaded place for her.
If you can't get a new fly mask for any reason:
Would she mind if you took a wet cloth to tie around the noseband of her halter so it ends up over her nose? It'd feel good on her burn and would protect it from the sun. I fear she may rub that off, but if you could put a hole on each end of the cloth and/or use something to tie it to the halter, she wouldn't be able to take it off. That is, if it wouldn't bother her too much for you to put it there in the first place.
If it wouldn't bother her, just take the cloth and dip it in ice water, when you put it on it shouldn't dribble, as that could annoy her. It'd be easier if you tie it to the halter before you put it on her. Check on it every once in a while, and add more cold water with a water bottle when it's too dry.
I've done this for one of the school horses I turnout when I had nothing else on hand to protect his sunburnt nose. His fly mask didn't cover his nose. And he was completely okay with it! A few days later when I had the time, I found him a fly mask that covered his nose to do the job and applied some Aloe Vera plant juice to his nose that my mom gave me(she's a gardener).
Try applying Aloe Vera, if you get it right from the plant, that would be the best! If you don't have the plant, use Solarcaine on her nose.
As "gallop" says, applying lip balm with sun screen to your horses lips will help too!
I wouldn't use bag balm since the petroleum jelly holds in the heat and will just cause more discomfort.
But I hope she feels better for whatever you do! :)