As far as I can see, twice the fare from Heaton Chapel is correct. The Railways (Penalty Fares) Regulations 2018 state:
"9.—(1) Where a penalty fare is charged under regulation 5(1) to a person travelling by, present on, or leaving a train, the amount of that penalty fare is £20 or twice the full single fare applicable, whichever is greater.
(2) In paragraph (1), “the full single fare applicable” is the full single fare for the journey in question from the boarding station to—
(a)the next station at which the train is due to make a scheduled call; or
(b)the station at which the person leaves the train if, at the time the penalty fare is charged—
(i)the person is leaving the train; or
(ii)the train is stationary at a station and there is sufficient opportunity for the person charged to leave the train at that station."
These Regulations make provision for the charging of penalty fares for the failure to produce, when required to do so, a ticket or other authority authorising a person to travel by train or to be present in a compulsory ticket area at a station.
www.legislation.gov.uk
There is no exemption for a passenger overriding on a ticket, which to me seems a little unfair because the pasengers is being required to double pay for a section of their journey, but unfortunately they are the rules. Getting this changed would be a separate matter entirely. I would suggest a letter to your MP might be a start in this.
It should also be noted that the passenger was traveling with their partner who did obtain a ticket from the machine, so could have assisted the passenger in obtaining a ticket.
I think the one thing in the passengers favour is that the TVM at Heaton Chapel will only sell a ticket from a remote station in conjunction with an itinerary. As the required ticket was from Blackrod, a station at which the train did not stop, an itinerary could not be produced for the specific journey required, which I can see could cause confusion. That said, it would be possible to obtain the required valid ticket with a different itinerary.
Other than that, unless there is any signage issue at Heaton Chapel or other similar technicality, I believe the penalty fare to have been issued correctly although I do sympathise with the passenger in such a case.