My wife is pretty much in the same boat, she is currently using a Specialized Myka fitted with slick tires, I also locked out the front suspension. At some point in the future we will most likely pick up a road bike for her as well, but for now the Specialized will do, and that way we don't end up wasting $300 on a cheap bike that will end up sitting in the back of the shed.
I say keep the MTB for now, if she does well in the triathlon and wants to take it up regularly, then invest in a decent road bike.