SEO Sai Gr. Hosp.
210628 Points
Joined July 2016
For AY 2017-18, your cousin stayed more than 181 days in India, and hence for IT purpose s/he will be treated as 'resident'
So, he/she has to file return in India (ITR1 not applicable in this case), and declare all his/her global income for assessment in India.
As the employer has already deducted tax from salary at London, s/he can get tax residency certificate, and based on that s/he can claim rebate under IT act.
For more details refer: foreign-income-of-resident-indian