The principal thing you need to clear up is the exact terms of the right of residency i.e. what exactly was said in the Will. You need to establish the position re: the following:
1. Right of residency doesnt necessarily mean that the person with it is the sole occupant of the property. It may simply be that you can set aside a bedroom for them and rent out the other bedrooms in the property. Generally speaking, the rights tend to extend arrangements that existed before the owner died. In this case, was the person the sole occupant before the owner died? Or were they simply lodgers?
2. How long the right lasts for. Can the person dissappear to the other side of the world for 50 years and then return and demand to live in the property? Or is it that the right of residency lasts only so long as they are in permanent residence. If it is the latter and the person has been away for more than a year, then you could take it that they are no longer permanent reisdents and so have given up the right.
3. Usually with a right of residence, if the person holding the right is not currently exercising it, the property owner can do what they want with the property i.e. live in it themselves or rent it out to someone else. You need to clarify that this is the case.