What is the name of the phobia of dams?
As far as I know, there is no specific definition for this phobia. No search reveals anything similar. It is important to recognise that simply defining phobias is not medically useful and does not provide further insight into human cognition. It would not be efficient to provide a label for all the phobias which exist. Generally labels are created when the population of people with the disorder reaches some threshold that specific treatments are developed. Your friend simply has a "specific phobia" of the situational type (also see Wikipedia entry).

I imagine treatment would be the same as for most specific phobias. Generally this involves a form of CBT known as exposure therapy or graded exposure.

If you desperately require a definition, I like hydrostructuraphobia - taken from latin word for structure.