And Mr Rude Sygyzy if I am wrong then OK, I will hold my hands up and beg forgiveness, but could really do without the unnecessary sarcasm and rudeness in your post.
I had read it that his earnings were as an expat (away from UK) and that as a UK citizen upon retirement (either now or in the future) would he pay tax. If he earned his money in a sandpit and then chose to live in same sandpit after retirement then he is not a UK tax payer. If he returns to use the infrastructure of the UK then he would. It would be treated as income at the time payment(s) were made and it is not relevant that you didn't pay uk tax on the income from which the pension was derived.
As it transpires (I assume) he is a foreign expat in UK so it will depend where he is resident and under which states tax regime he pays tax at that time. So, as an expat in the UK he will be treated as an individual with significant UK connections and taxed by uk authorities accordingly. If he lives away from UK by the time he can withdraw his pension funds (over 55 at the moment) then it is more complicated and will likely depend on wether he took advantage of the tax free status of pension contributions at the time funds were paid in.