Is the accountant sinful for entering data for illegal trade visas via the same computer used for accounting programs, especially since he does not receive payment for this, and he has advised his sponsor without success? And is his salary considered unlawful due to suspicion regarding the sponsor's money?

Selling visas is not permissible except for the cost of their issuance and a fair wage, because it is a contract of donation. Therefore, it is not permissible for your husband to assist the employer in that forbidden act. He must advise the employer and explain to him the impermissibility of that. If he finds no alternative but to cooperate with him, then he must leave the work unless he is compelled to it until he finds another job. The mixing of the employer's money does not affect the permissibility of your husband's salary as long as his work is permissible.