Income tax return for 8months salary from overseas vietanam country

My Clients is Resident Indian, Regular Income Tax Return ITR1 Salary Return filer. 

1. In FY 2018-19 his Indian Office depute him overseas Vietanam country for 8 months.  He will be receiving 8 months salary from Vietanam.   

2. In Vietanam he will be paying 35% Income Tax on Salary.   

3. How will be Income Tax treatment in India. 

4. As in FY 2018-19, his 8months Salary will be receiveing from Vietanam.    Whether he will have to file Income Tax Return in India OR Vietanam ?? 

5. How will be Income Tax treatment.   From Indian Income Tax matter what precautions to be taken, 

Please advise.....