What right have you to make such an entry in a personal log book? You can certainly give the candidate advice, and refuse to rent them an aeroplane, but using a log book as a "training record" could come back to haunt you if the pilot was subsequently involved in an incident.
Every right, if invited to write in it.
I don't know the circumstances of this specific event and wouldn't wish to guess but should I find myself in a position where I was not happy with the performance of the pilot seeking revalidation, they would be given the option of either me not signing anything (i.e. the pilot will then need to fly with someone else anyway in order to be revalidated) or putting a suitable comment, such as the previously mentioned "further training recommended". I don't have licence signing privileges but equally I would not want to put an examiner in the position of signing a licence on the basis that "Tay Cough was happy" when I wasn't.
I assure you a world of pain will befall any instructor who revalidates a pilot despite having had major doubts over that pilot's performance if that pilot subsequently becomes involved in an incident.