Monterrico gets a lot of visitors on the weekends -- it's about 90 minutes south of the capital -- but it's pretty quiet during the week. It's also close to the border of El Salvador, so Salvadorans make up a lot of the visitors too. Black-sand beach. Small selection of hotels. This is not Puerto Vallarta by any stretch of the imagination, but it's an interesting little place. It's basically a hot, sweltering little beach town.
I think it's reasonably safe with all the standard precautions. Don't go walking alone on the beach. The few streets, which are really mostly dirt roads, get pretty dark at night, so she shouldn't go walking alone at night. The surfing is good, but, conversely, that means swimming is risky.
If she's doing conservation volunteering, I take it she's working on a project with sea turtles?