It is not safe but I think he can't move funds out of the exchange without email access. But he/she can make nasty moves if there will be misunderstanding between you two and you can easily loss so much or even all you have if he/she will Buy/Sell some shitcoins at ask and bid price which has high price spread like of 10%.

So I will never give access to my account to anyone even not to my brother or best friend.