Does he have to pay tax on the income earned by him abroad, which is paid by the Indian company?
Certain income is deemed to accrue or arise in India even though it may actually be received outside India. Since he is employed with an Indian company and has been deputed abroad, the payment made to him is deemed to accrue and arise in India. So he is liable to pay tax on this income. If he joins a local company abroad, then he is not liable to pay tax on that income.