Umbrella Company

a friend drives a taxi on the weekends for additional income. he is also a contract engineer being paid as an employee via an umbrella company.

recently, he was advised to register as self employed in order to put in self assessments for his taxi income

His umbrella company have now advised him that he will have 20% deducted (which i assume is CIS) and have to pay over his NI himself.

This is confusing as registering for self employment to pay across tax on the taxi should not alter his sitaution with the Umbrella company.

please can someone advise

thanks