A customer who is liable to submit a tax return where he/she has a tax residency but has not submitted a tax return, should submit a tax return afterwards. If a person voluntarily submits a tax return via a posteriori (self-correcting), any penalties are normally lighter compared to if a tax authority discovers any failure to submit a tax return and pay taxes.
Credit of tax, to avoid double taxation, may in some cases be allowed for income taxed in another country. This depends on whether there is a double taxation treaty between the countries in question.
We should recommend the customer to contact a tax advisor in case the customer needs assistance with correcting a tax return or has questions how this procedure works. Generally, we may recommend any of the major accounting firms (PwC, EY, Deloitte, KPMG) as they usually have local contacts in all countries and can handle any issues that may come up because of the tax liability in another country.