It depends on who employed and paid the specialist during that period. The requirement is one continuous year of qualifying employment with the related company abroad within the three preceding years, and a secondment to an unrelated party can interrupt it.
Count the months the employment relationship actually held
Hypothetical example: the specialist worked for the Coaldale company for four years, but for six months of the last three years she was placed with a joint-venture partner. Two facts decide the outcome. First, whether she remained employed and paid by the Coaldale company throughout, with the partner receiving her services under an agreement, or whether she was employed by the partner.
Second, whether the entity she worked for during that time is itself a qualifying related company. If the employment relationship with the Coaldale company continued unbroken, the year can still be established with payroll and the secondment agreement. If it did not, the file may need to wait until a fresh continuous year has accrued.
Do not describe the period vaguely to make it fit. The other requirements remain: a qualifying relationship with both entities doing business, and knowledge of the company's own product, process or procedures rather than general professional skill.