In English law - they would have to show intention to defraud which he may not have - his obvious state of mind wd be that he forgot or didnt know ( negligence ) But if they convicted - then the fine wd be the amount of tax owed - it unlikely he wd be imprisoner for this
.There is the little matter of unpaid tax - this is civil - so he wd have to pay the unpaid tax and the extra charges of not filling out the form.
there are also pains and penalties up to 100% of the unpaid tax - and this is likely to be highish - 80-100% because he did not make a voluntary declaration, did it over a few years and concealed
Triple trouble for not paying tax - dont do it
even in you mind