It sounds like either some form of depression, or lack of confidence. You could try to get them out more, introduce them to new people and get them active. Sometimes people get into the rut of not doing much, then they find it harder to make friends as they have less to talk about. Breaking this negative circle would be a good way to help your friend. Introducing them, bringing up fun/funny moments you have shared, getting them active ... will all help break them free of this feeling.
Hope this helps your friend.