Does she have braces?

The echinacea should be okay. It is best to use it in a pulse fashion.
Like 21 days on, 7 off. It is only a serious problem for people with autoimmune
disease.

There are lozenges called Aqua Drops ...that keep the mouth moist for a short time (probably wouldn't work all night)...she could suck on one before going
to sleep. (make sure it is all gone before falling asleep). Most pharmacies have them in the candy section. (not commonly sold in grocery stores, however).

Also zinc is very good to improve cell resistance to viruses and infection.
Many people are low in zinc. This can be done with Cold-Eze lozenges during the day (3 or so), or an oral zinc supplement like OptiZinc 30mg once a day.
Zinc also helps with acne which teens struggle with.

Make sure she washes her hands alot... going to school, shopping you pick up tons of viruses on your hands from surfaces.. and you can transfer them to the throat in a second, and spread them around the house too, by just touching things.

If white patches appear in the throat.. you should check with a doctor--
might need antibiotics. Chronic sore throats in teens can also be Mono...
a quick blood test will show that. (my son brought that home from college one year and he got a secondary throat infection from it that was horrible).
Mono itself can cause a long term sore throat.

When I get a viral throat I spray Chloraseptic on it...the liquid has phenol in it and kills germs for most mild to moderate issues, I find.