Thank you, Maths. So, if he is not resident in UK for 17/18, which I think likely, the gardening leave for the 90 days he spent in UK will not be taxed in UK at all?
Not quite. Non-residents are subject to UK income tax on salary attributable to UK duties [I'm assuming here that although no actual duties are performed during the gardening leave period, for the period spent in the UK this will give rise to an income tax charge; haven't had time to look at the legislation].
However, if he is resident in Kazakhstan for the period spent in the UK then the DTA would potentially offer him protection from a UK tax charge which would require the employer to be non-UK resident. I note you state employer is in fact UK resident.
Bottom line, UK income tax applies.