Thanks for the quick responses!
I was going to buy her a return ticket (hopefully 2 one-way tickets counts as a roundtrip even though they're not the same itinerary), but if we don't know how long CBP will grant her, then how long should we plan on her staying in the US? If I buy her a ticket a couple weeks from the 6 month mark and they only grant her a 2 month limit then we'd have wasted our money.
We would like to maximize her stay within the limits, but it's hard to do that if we won't know the limits until she arrives at CBP.