I was in a similar situation a few years ago with my friend and I remember how hard it was to get practical advice. This treatment is not like many others as it will involve a long spell in isolation.
My main advice is to be prepared for the long haul. Don't overcomit yourself, run out of steam and/or let the novelty wear off. She'll notice. So in terms of keeping in touch and sending her things don't be tempted to send her messages everyday or buy her loads of things up front. Spread it out as she may be isolation for weeks (maybe even months). Giving her things to look forward to will be a gift in itself.
She'll be poorly, tired and will probably have a low attention span. She'll also be regularly checked (and interupted) by the medical staff so she probably won't be able to cope with anything too involved or taxing.
Magazines are good to dip in and out of. Story CDs are good too (nothing too heavy going and make sure she can play them!). Send a mag a week and maybe add a handwritten note saying which story or section you liked. It may also help to know the story cd you're sending her so you can chat about it, the same if she has access to iplayer - try to keep up to date with her favourite programs to chat about or to fill her in if she misses them. This is as much for you as it is for her in case you worry about running out of things to say and if you're feeling guilty about talking about the outside world.
Consider kids craft activities for her e.g. make your own braclets, colouring in etc. This aren't very taxing but will give her something to do and take her mind off things as well as giving her something to show for it at the end. They are also fairly cheap to source (supermarkets and The Range etc) and easy and cheap to post.
If you do post anything the outer layer will be considered contaminated and will be thrown away. So if you want her to receive pretty packages and letters in envelopes then just double wrap everything.
Her appetite may be affected and her skin may feel extra sensitive so speak to the unit for advice on what's suitable to send. I remember calling the unit and I think also the Anthony Nolan Trust. I don't think they were able to help much because there isn't really much you can do at this stage.
You probably already know that she'll be limited to 3-4 named visitors who have to shower and change clothes each time they visit the unit. Keep in touch with them for feedback. Sometimes supporting the supporters is all you can do! It may be that you and other friends will need to take over once she's realeased as that's another huge hurdle and long journey.
One really important thing you could organise is for her home to be given a really deep clean. It will help her emotionally as well as keeping her safe. My friend was given two main pieces of advice when she was discharged - she, and everyone around her had to wash their hands really really well. After going to the loo, before preparing food and eating. It's so simple but so important. And I mean properly, we put those cartoon posters up to remind people! Also, everyone who comes in to contact needs to be in good health to the point that if she lives with someone they may have to move out if they are ill. The risk of going out and coming into contact with sick people and snotty nosed kids could mean that your friend effectively returns to isolation once she's home.