The visiting instructor has the same "right" to offer a rank as anyone else. As I mentioned in an earlier post, any rank credential is just somebody offering their assurance that a person meets certain standards within a given art. The question is - what is that assurance worth? I could issue someone a rank certificate in karate, but it would be meaningless since I have no significant background or expertise in any form of karate. I could issue a rank in BJJ to someone that I just observed at a seminar. This would be "legitimate" in that I have significant BJJ experience and my expertise to make such judgments is recognized by the BJJ community. However my assurances on the matter would be much less informed than they would be if the practitioner was a personal student of mine. Even if I knew a BJJ practitioner and their abilities well, I would not offer to promote someone who was an active student of another instructor. Not because the rank would be invalid, but because accepting it would cause friction between the student and their teacher. Best to have one person responsible for tracking the student's progress. In some systems, it is common practice to have students tested for rank by a panel of senior instructors from outside their school. I can understand the arguments for such an approach, but I think it leads to a less informed judgment of a practitioner's capabilities than promotion based on an instructor's day-in and day-out observation of the student's performance.