... Is that breaking a regulation of some kind, and if so do you have a reference for that.
It seems to me that a logbook is sort of like a diary, you can write what you want as long as that time is not being used as a fraud to qualify for certificate by entering that time on an 8710 airman application.
Yes and no, IMHO the devil is in the details.
If (in this case) a student pilot makes an entry into their logbook (took a flight with Grandpa for over an hour) and does not count total time, PIC/solo time, or Instruction recieved - IMHO he is not recording flight time but a personal note. The log book is a legal record for recording flight time for currency and time for a certificate or rating (CFR 61.51).
If he uses that time for total time, that IMHO can be used for both times above.
61.59 Falsification, reproduction, or alteration of applications, certificates, logbooks, reports, or records.
(a) No person may make or cause to be made:
(1) Any fraudulent or intentionally false statement on any application for a certificate, rating, authorization, or duplicate thereof, issued under this part;
(2)
Any fraudulent or intentionally false entry in any logbook, record, or report that is required to be kept, made, or used to show compliance with any requirement for the issuance or exercise of the privileges of any certificate, rating, or authorization under this part;
And one more point. When a student pilot does any thing the FSDO questions, the FAA will investigate the CFI to determine their competency to be a CFI. Most CFI do not get paid enough to keep a lawyer on retainer and IMHO, it is best to travel inside the boundries of the regulation than to live on the edge and maybe cross over.......