My, non expert, advice to your friend would be to contact the tax office after Coronavirus is all over, explain his situation and ask for a deal. I did this many years ago: after I came back from travelling in my 20s I worked for cash for a while but decided I needed to get back into the system. They gave me an emergency tax number for a year or so and a bill for NI which I also payed over a year or so. They are very happy to welcome back tax payers into the system and therefore very helpful well at least they were in the 80s - circumstances may be similar now with high unemployment after Coronavirus.
Best wishes to him
My mate worked for ages as a builder then helping out a joiner. This was when he put his hand up inside a planer to clean it out and cut all the fingers off his right hand. This made pushing a barrow rather problematic ..... so he packed it in and got a job in the tax office!
He ended up being the 'Share Fishermans Telephone Helpline' for most of the UK!
He related that a typical conversation might go :
" Moi Missus reckons I been a roight Keuuuunt" ( not the way I'd start a conversation to a Govt agency, but hey)
" She reckons now the babbies come oi gotta sort out moi paypers"
" No oi aint never kept no recccrds loike".
And so on .....
He reckoned that his Boss said , "Just ignore what has happened in the past, you can't chase fresh air, just get them in the bloody system from now on".
Lets face it , in the past most of it probably went over a bar anyway , so tax, duty and VAT would end up being paid , albeit in a rather roundabout way.
So based on that and the fact that they are now working with much reduced numbers of staff I think your mate might well be OK. I think they are a bit busy at the moment to be prosecuting people.
Just my 3 pennorth.