An electrician on one of our recent job sites was telling me that rigid steel is the only metal conduit NEC-approved for exterior use, but I haven't read the code myself, so I won't defend this.

No, not true. EMT is and always was approved for outside however most EMT "water proof" compression connectors have recently been DISapproved for outdoor use because they leak like a sieve. There are new approved connectors available incorporating O rings but they are not easy to come by.

So I guess your electrician is correct in a way. Unless you use the new connectors you can't use EMT outside, at least for class 1 wiring.

-Hal

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