How to resolve the dispute

Dear all,

I have come across a case where as person is staying in a property for 15continious years and has legal docs like Ration Card, Passport, Adhar Card etc. The ower of the flat is no more and has no legal hiers.

In this case does the flat belongs to the person staying there for 15years. Will he have the ownersip rights or only Possession administation rights. Does the society can declare him as a tresspasser