IS MLR required for foreign tax evasion

Without going into too many details, assume a client is UK resident, and completely compliant with UK taxation. However it is has come to light that he is also apparently liable to pay tax in a certain foreign country. The client refuses to file a tax return in the foreign country, and obviously does not pay the tax that would be due if he did so.

Is a MLR required to SOCA? Would it make a difference whether there is a DTA between the UK and the other country? Would it make a difference if the other country is a fanatical dictatorship or is a "Western" country? Would it make a difference if under the laws of the foreign country there would be a requirement to make an equivalent report?

I am asking strictly for the "legal" perspective, and whether a report is required to be made to SOCA, not to the foreign country.  I already know (in my opinion) what is the correct "moral" answer.