It is a legal requirement to buy in advance, it's not just recommended. FFS, just because you don't like the advice being given doesn't mean it's not correct.
www.nationalrail.co.uk/static/documents/content/NRCOC.pdf
Page 4:
You must:
• Have a valid ticket or tickets for the whole of your journey, at the time you want to travel and on the route and trains you want to use. Unless otherwise indicated, you must purchase your ticket before boarding the train you intend to use.
Page 5:
If you travel in a train (a) without a ticket; or (b) the circumstances described in Conditions 10, 11, 12, 18, 19, 22, 30, 35 and 39 apply;
you will be liable to pay the full single fare or full return fare or, if appropriate, a Penalty Fare (see Condition 4) for your journey. You will not be entitled to any discounts or special terms unless either:
(i) at the station where you started your journey, there was no ticket office open
and
there were no self-service ticket machines or no self-service ticket machines were in full working order
and
in Penalty Fares areas you bought a Permit to Travel unless no Permit to Travel issuing machine was in full working order
OR
(ii) the notices and other publications issued by the Train Company in whose train you are travelling indicate that you can buy tickets in that train.
In circumstances where (i) or (ii) apply, you only need to pay the fare that you would have paid if you had bought a ticket immediately before your journey.
Sapphire, the buying before he boards thing is critical in this case, it's not finger wagging, it makes a huge fucking difference. If he could have and chose not to he hasn't got a leg to stand on which is why I've asked if there is a ticket office at the station and whether there is also a machine.
To be blunt, I don't think OP has all the details off her DH. And that doesn't make her or him guilty but there's a reason this sounds implausible to me, and to others. The issuing of a fine on the train by the conductor which could be paid later is inconsistent with the conductor's machine not working, which is why I mentioned UFN which is what this could be. If OP doesn't want to give any more info that's fine but she won't get any help from railforums without it either.
And OP, I know a hell of a lot more about the railway than you clearly do!