2. Does she have to register to live/work in Poland (she'll be staying 6 months or more)? How? How much does it cost?
Yes, she needs to register with the tax authorities and obtain a residence permit. How? It's all covered on the forum. Cost? 1zl.
3. Would she be eligible for health insurance/retirement from employer?
No, unless she has an umowa o praca/umowa zlecenia contract.
4. What's the minimum wage for foreigner workers?
Gross - 80% of 1,317zl in the first year of employment, per month, based upon a 40 hour week. Net? Depends.