Supplying goods to you at a profit is a trade. He supplies you with an invoice (with VAT if applicable) and you pay that.
For him, he will be taxed under Schedule D Case I if a sole trader and if it is a company, it will be Schedule E (if salary) or Schedule F (if dividends). Either way, its his problem and he needs to do accounts, tax returns etc.
From a Revenue tax point of view, there is no issue here. However, from a commercial point of view, this is the real issue. Will his employment duties suffer, if a employment issue arises (or a supplier issue) how will this be resolved so as not to affect the other?
Also, watch Revenue salary sacrifice rules. Is this a genuine commercial transaction at arms lenght whereby salary is not being sacrificed so as to avoid PAYE/PRSI?