The best I can say is adjust diet and hopefully the weight will get down to where your friend can move again.
I have known someone that was really big, but they could at least get out of the bed.
I think that having your friend do some stretches every day is key. An hours worth of stretching can really help a person who is that big, because they tend to get stiff just laying or sitting all day. Even stretching while they are sitting in the bed can work wonders.
There are a lot of exercises DVDs available for people with limited mobility. These usually contain exercises that can be done from a chair, but you might be able to adapt some of the moves.
The best advice would be to speak to a doctor or therapist who can best advise you. However, there are some clips on YouTube that show exercises for bedridden patients which would give you some idea of where to start.
This article here also gives some advice, but the exercises are not illustrated.
Thank you all so much for the suggestions here! I used to be an avid runner, but I have a broken foot right now. While I'm not necessarily bedridden, I'm not exactly mobile, either. I try to get my heart rate up using my crutches, but it is exhausting. I've been looking for more exercises I can do for bed, and the suggestions here are excellent. Since I'm not working out like I would like to be, I have been very concerned about packing on extra pounds. I think this will, indeed, help.
What may help the leg lift is to use a long rubber band as an aid, found in fitness stores, or even a regular men's tie. You hold both ends in either hand and loop the tie around the sole of your foot. That'll help her get started on the leg lifts, without expending too much energy until she's ready to do it without the aid.