If he gets caught, he's screwed, and rightly so.
Deliberate, false entries in a logbook constitute fraud with the aim of obtaining a qualification which will put the perpetrator into a position to be responsible for the lives of hundreds of others.
The CAA - rightly - takes a very serious stance towards this.
I have heard of a story where a falsely logged cross-country flight was detected by the CAA at the time of licence application, and the pilot's whole licence was revoked; his further career as a pilot found an abrupt end.
Further, I quote from a CAA application form:
It is an offence to make, with intent to deceive, any false representations for the purpose of procuring the grant, issue, renewal or variation of any certificate, licence, approval, permission or other document. Persons doing so render themselves liable, on summary conviction, to a fine not exceeding the statutory maximum (currently £5000, or in Northern Ireland £2000) and on conviction on indictment to an unlimited fine or imprisonment for a term not exceeding two years or both.
In other words, your knowledgeable friend, who knows his stuff for sure, might get some quality time to reflect on things in prison.
Your friend jeopardises himself and others - future passengers and fellow wannabes, who have to jump through the same hoops to get their qualifications.
I hope he has the sense to correct his logbook before he sends it in.
KK