In general, AmEx cards *can* be used for foreign transactions. I use mine all the time. I happen to have an AmEx card that does not charge a fee for transactions in foreign currencies; I just pay the interbank rate on those purchases. This can be a non-trivial savings on bigger trips/purchases.
Some vendors will not affiliate with AmEx because their merchant fees are higher. Even when a foreign vendor will accept it, it might be declined by AmEx unless you've informed them that you plan to be abroad---it depends on your precise spending habits, and what the bank thinks is your "normal" spending pattern vs. what might be fraudulent. And, it sounds as though in the OP's case, they are trying to use an employer-provided card which restricts the things for which the card can be used as a matter of policy.