If her sexual partners are asymptomatic, vaginitis may be more likely than an STD. But if not - or if this fact cannot be ascertained - then STDs must also be kept in mind.
Putting STIs aside, there are many type of vaginal infections a woman can have and treatment for each differs. Excluding STIs, the main types are fungal (candida), trichomonal and bacterial. There are some "3 in one" preparations to be had which can be useful when it is nto clear which type is present, as follows:
Gynecon
Neo-Penetran Forte
and some others. No presecription needed.
These are vaginal suppositiories, to be inserted before bed. Use up the full pack.
Please ignore the suggestion for Amoxicillin Clavulanate (clamoxil) as this is not effective treatment for vaginitis and indeed can make some types worse.
If above treatment does not work then there is nothing for it but to see a doctor. Ditto if an STI is suspected.