Must see locations include Manuel Antonio National Park - this is for great wildlife along beautiful beaches, and Arenal National park - this is an active volcano, rainforest, and geothermal features. Other things depend upon her interests. The Poas volcano can also be visited along the way to Arenal.

As far as renting a car - it all depends upon how comfortable she is with unfamiliar, and pothole-strewn roads. I rented a car and drove to the aforementioned locations. Coming back from Arenal to San Jose, I hit a huge pothole and got a flat due to a dented rim. Fortunately, my spare was in good shape, and I didn't hit anything else on the rest of the way back. If renting a car, make sure that damage insurance is gotten.

For accommodations, one can get basic ones for $50/night, or ultra-luxurious ones for $300 a night, or anything in-between. As I was there several years ago, I would hesitate to make specific recommendations, because what was good value 5 years ago might be overpriced or poorly maintained nowadays. She'll need to fly into San Jose, and make that a base for 2-4 day trips to the places I mentioned as well as others.