Is she plucking eyebrows or facial hair?? There is a big difference.
If she has thick facial hair she may be producing too much testosterone(male hormone) but if it's just eyebrows then no-one i've ever even heard of has had a problem with that growing back thicker (she may just notice it more because it's been plucked?) although as we are all unique, hers may grow back thicker.
I use my fingers to pluck lose eyebow & eyelash hair (they come lose mainly around period time) & that's never caused me any problem.