Your friend will have no problem rushing as a junior. MSU is a commuter school, and a LOT of the students are older. I know girls who rushed during their junior year after going to Community College. There were also seniors and fifth years. One of my friends actually pledged while living at home. She had a cerfew of 12:00am, and she was always allowed to go home on time.
Formal rush is in the fall and informal is during the spring. Most girls rush in the fall. PM me about this if you want to know why.
MSU has 4 NPCs: Tri Sigma, SDT, Phi Sigma Sigma, and Delta Phi Epsilon. I remember 3 locals... Sigma Delta Phi, Theta Kappa Chi, and Delta Xi Delta (?). There may be a 4th. There are no houses, but there are floors reserved for Greeks.
Personally, I definately reccomend going greek at MSU. As I said, it's super commuter oriented, and (in my opnion) the best way to really make college a great experience there is to go Greek. I don't usually say that, but I think it's true.
Please PM me if you have any other questions about Montclair, and I'll be happy to answer them