If you want to make sure that you are going to get your injured friend something great for their birthday, one thing you can do is think of something that will help them feel better. Most people who have bad injuries have a hard time sleeping, but sleeping is one thing that they absolutely need to do in order to get better. If your friend does not already have something comfortable to sleep on, then you might be able to help them.
One thing you can do is find them a malleotrain to help them support their ankle if this is where their problem is. Not everyone with an injury has a problem with their ankle, but many people do. If your friend is one of these people, then you can consider trying to find them one of these products that can help them have support while they are walking around each day. You may not realize how much that little bit of extra support can help them.
Another thing you can do is consider getting them a mattress topper. These can be very useful because they can help your friend be more comfortable as they go to sleep each night. This will be a great thing for their body because they will be able to heal much faster than if they did not get this good sleep. Mattress toppers can be relatively inexpensive as long as you find a trustworthy source to get them from. This can be a great thing to do especially if your friend does not have a good thing to go to sleep on. If they can get the deep sleep they need, then they are going to be much more grateful in the end.
If your friend has a sore back and not an injury, then you might want to consider getting them an inversion table. If they have never tried one, they may not know what to do. Basically all they have to do is strap their ankles in and hang upside down. The table can help them to heal by extending their spine. You just have to make sure that they do not have any injuries that would be contraindicated for the use of an inversion table. Inversion tables can also be rather affordable.
When you are trying to find something for a friend who is injured, you just have to remember that they need to sleep and get well. If they have had the injury for years, then you might want to just get them something like a coffee maker that can be useful for their day to day life. If their injury is rather new, then they may not have the things they need like malleotrain products or mattress toppers.