Something like this?
UPDATE employee
SET employee.emp_salary = employee.emp_salary * 2.0
WHERE EXISTS (SELECT c.EMP_ID FROM customer c WHERE c.emp_id = employee.emp_id)
[Edit]
UPDATE employee
SET employee.emp_salary = employee.emp_salary * 2.0
WHERE EXISTS (SELECT c.EMP_ID FROM customer c WHERE c.emp_id = employee.emp_id AND c.cust_id=4)
I don't know Oracle, so the above is a general solution. E.g. with MS SQL you can use left join in updates which would be probably more performant.