Q: "She became involved with a married man [who is] many years her senior." - Is "who is" required for correct grammar? 
She became involved with a married man many years her senior.

Is it correct in grammar?
I suppose it should be 

She became involved with a married man who is many years her senior.

Is my opinion correct?

A: She became involved with a married man who is many years her senior.
"Who is many years her senior" is a relative clause. I think You can reduce a relative clause  if the relative pronoun that, which, or who is the subject of the clause and there is a linking verb "be".  In such a case, you can omit the relative pronoun and the verb form be (is, are, was, were, etc.).
So the sentence without "who is" is also correct grammatically.
