Tax Implications for Salary from Overseas

From Indian employee perspective, he can consider going for volunatry PF registration. However, not sure whether US MNC would agree for it. He can check with his employer. On a separate note, there is high likelihood of US MNC having PE in India unless it is a Liaison Office. Further, since your cousin is Indian resident, his global income is taxable in India. Hence, in the instant case, the salary is taxable in India. 

For claiming the exempt component of Salary, you need to check with the employer. If the employer provides those components, exemption for such should be available. Please take note of perquisite taxation as well.