"Our grandfather speaks perfect" is a correct english sentence. Just like "She already walks perfect in those heels" or "The child is already talking perfect". Its very commonly used like that and should be accepted.
It is grammatically incorrect in English. When describing a verb you should use "ly" at the end of the word. As in "drive safely" or "speaks perfectly". Although, it is common to hear English speaking people not use this rule.
you know how "nødvendig" becomes "nødvendvis?" My initial guess was that "vis" is the danish analogous word ending for "ly" in english. Why doesn't "perfekt" become "perfektvis" and is "vis" even the analogous word ending?