Rules for mobile phones/laptops/internet can vary between units, but you will be able to contact her - if you can get the address of the unit I'm sure she would love to receive some post. I got a lovely book with messages of support from some of the lovely people here when I was inpatient, and it really lifted my spirits.
Whether or not she will get back to where she was before she slipped is much harder to answer. She might be able to, but she might also struggle for a long time, and sometimes that's very difficult to break. But with the inpatient treatment she is receiving, if she is able to work with them and really get the most out of her treatment (even if not initially, then during the later stages of her inpatient stay) then she has good chances of getting her life back.
If you aren't always able to contact her, then the best thing you can do is keep doing things you enjoy. Try to see other friends if you can so that you don't isolate yourself, put your own needs first and keep talking to people.