First of all - it may not actually be yeast - it could be irritation from perfumes, dyes, toilet paper, soap, laundry detergent etc. all need to be eliminated - plain WHITE COTTON panties, ALL free and clear, dove soap - wash underwear separate - rinse twice... no pantiliners - those make it worse especially Always brand - just change panties if moisture is an issue. Those wipes are awful!! A definite no, no if you have chronic irritation! She should rinse with water and pat dry - or honestly air dry (or use a hairdryer) - sounds weird, I know.
Then if it is truely yeast - she should be tested for diabetes - as many have suggested! She may need to be on an oral antifungal for 6 full months - I know - I have been there!
Balance the pH - RePhresh is good for this (over the counter) - she could be dealing with chronic bacterial vaginosis too - which is a total pH balance issue - so make sure it is really yeast - or yeast treatments will actually just worsen the problem!
Probiotics - definitely!! Agree with all that have posted this!!
Good luck - feel free to pm if you have more questions.