In case, the basic is above the PF limit, the employee can opt out even if he was contributing towards PF in previous place of work. Same applies to employees who cross the threshold limit due to pay revision.
Check the form to be filled--I think it is PF Form 11
Please go through with PF act, clearly mentioned Under section 26 para 3
if he was availed that benefit in previous company then he must contribute with current company(if he not withdrawn the money). and as P K Sharma suggested that you can restrict his contribution upto ceiling point 6500 /-.