Adopting an adult

Hi All,

A friend of mine lost his parents to a road accident 6 plus years ago. Since my family and his were friends from before that incident my parents had a parents-son bond with him. Now they want to legally adopt him and provide him with all the benefits that the law can provide. This friend of mine is 28 years old, while both my parents are 53+. How do I begin this process? Is it legally possible to do so? Are there any constraits? If so, then what is the solution?

Anxiously waiting for some help to my doubts.

Thank you,

Anamika