So was she diagnosed with MRSA? Or is this a run-of-the-mill staph bug? If MRSA, it's not surprising it's coming back.
Also, staph is pretty much always present on us and some of us are more susceptible to staph infections than others and get infections of one kind or another on a recurrent basis. Other possible reasons: these infections are not getting completely killed off and the bacteria sticks around (possibly stronger bugs) to reinfect.
So no really helpful explanations - just what little I know. I sure hope she doesn't have to deal with this over and over and over going forward. What do the doctors say?
Also, what antibiotics do they give her to treat this bug? You could always ask if they would be willing to give her one of the big gun antibiotics (Vancomycin, for example) to see if it resolves the current infection AND prevents future infections for a while. They usually hesitate to use those powerful antibiotics but after three infections maybe they would. You need to make sure and ask about side effects of all antibiotics though so you are sure you are okay with them.
Steve