I have heard that occasionally such clauses are written into custody orders, but that usually happens at the beginning of the proceedings. After an order is set, it would be unlikely that you could get such a clause inserted as an addendum.
The question has two sides. On the one hand, you want to protect your child from any dangers posed by the friends of the other parent. On the other hand, merely asking the questions makes you appear as though you are trying to be overly "controlling."
Don't get me wrong, I understand your feelings. I think we probably have all had them, to some degree. It's probably best to try to discuss this with your X first, in a non-accusatory manner, to see if you can come to any agreement.
Good Luck.