We don't know which state user is currently in when we send an invitation to a friend in another instance.
That makes sending req invite passive.
Even the receiving part cannot deliver the message "I can't invite you because I'm in the only room" to the person who sent me the Invite when User was in Friends Only, or Invite Only.
Therefore, I think it is necessary to disable the button so that people in Friends Only, or Invite Only Instance can't send req Invite, or, when I send Invite, to tell me which Instance the other person is currently in, along with a Decline message.
I think this is a good way to keep users from having more communication problems in VRChat.