Purchase should come after they have started some type of specific exercise program on their own without these devices. Starting and keeping going with a simple walking program for several months will give you an indication if you are going to consistently stay on some kind of an exercise program. Several weeks ago I wrote a paper on recumbent bikes stating they are bad for people with low back pain. I got numerous calls and concerns about this and as I told many people, it's not that all people should not use recumbents, it is just a matter of that person's specific problems. The pieces of equipment that you choose must fit not only your needs but take into account physical problems.
Treadmills are one of the more common pieces of equipment bought.
These are very good pieces of equipment unless weightbearing types of activities bother you. If you have a degenerative knee or degenerative hip, this is a piece of equipment you would want to avoid. In this case I would look into an Exercycle or a non-weightbearing type of device. This would also apply to the newer elliptical devices on the market. The other thing I have seen a lot of is the different types of abdominal conditioning and strengthening devices.