It is an interesting problem but as you get into the details it rapidly becomes more complex.
- You need to detect the difference between a normal movement such as walking or kneeling and a fall. This is complex because we move by deliberately off balancing ourselves. This also has to be highly accurate, a 1% false positive rate would make walking around impossible.
- The person has to be saved from the fall. It isn't enough to straighten the legs, in fact that could make it worse. The whole body has to be righted, with assistance because the core muscles are probably also weak.
- As outlined in the original post, the righting will probably be performed by shifting out a counterbalancing mass. This mass has to be carried around and probably needs to be around 15-20% of the person's body weight. They obviously won't be able to carry this so assistance needs to be provided
Exoskeleton suits are awesome and I want one when I am old but often the simple solutions like a walking frame or stick are actually the best ones.