The American Heritage Dictionary of the English Language* has this definition under "falls" meaning "waterfall":
falls (used with a sing. or pl. verb) A waterfall.
In your reported conversation, either "is" or "are" go with Falls, except when the predicate nominative, in this case, "name" is definitely singular.
You might use the plural verb if you think of the Falls as coming from more than one place, as I think both the Canadian and American Falls do at Niagara Falls; that is, there is not just one big stream to be seen, but several distinct places over a wide area that the water flows over. _______
When you think of the Falls as one place, as an attraction like the Grand Canyon, you could use a singular verb, as in these examples:
Perhaps the possibility of a plural verb with Niagara Falls, and the Canadian and American Falls, is due to the fact that it covers quite a wide area and could be considered to consist of many falls. A waterfall such as Angel Falls, on the other hand, is very tall and narrow, and makes one think of one waterfall, rather than several together.
However, the word "falls" alone, without "Angel," referring to Angel Falls, does have a plural verb. _______
A search on the internet showed the name of Iguassu Falls with a plural verb:
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