Is the money an employee receives permissible, in return for providing technical and advisory support to an external company and informing them of his company's tenders, with a guarantee that he will not favor any company or interfere with the awarding of bids?

If the colleague's work with the second company is outside official working hours, and it is unrelated to the awarding of tenders, then there is no harm in him taking a commission. However, if the work is during official working hours, or if he is working in his company to award a tender to the company he is collaborating with, then it is not permissible for him to do so.