The main considerations when thinking about security for the elderly or vulnerable very much depends upon their individual circumstances as a person with mobility issues may have slightly different needs to a person who can get around. For instance, having a key safe box on the wall outside where only trusted people know the code may be necessary if the occupant cannot get to the door.
Having secure doors and windows is an important aspect of keeping people safe in their own homes. A spy hole fitted to the door so that the occupant can see who is outside is advisable or a doorbell camera will provide the same protection. Good quality locks on doors and windows are essential as is a door chain.
Fitting a CCTV system is a great way to give a vulnerable person some peace of mind as they can keep an eye on what is going on around their property and can alert the police if they see anything suspicious happening.
There are lots of agencies out there that can advise and support the elderly about security issues in and around the home and it is certainly worthwhile arranging a security assessment if that is appropriate.