If he says of her "she is a formidable woman" he must be using the term in its English meaning. But then, "formidable" in English may imply several concepts: awe-inspiring, fearful, intimidating, and a mixture of all that.

When he is exclaiming "c'est formidable !" he is definitely using the word in its French sense of "great, grand, wonderful, fantastic, ...".

But when Monty Python are singing "our shows are formidable" what do they mean?

Oh, and about the difference between "formidable" and "magnifique". Well, "formidable" in French may also mean several things, closely related: "astounding, extraordinary" (that's the closest you'll get to the English "formidable"), or more informally "terrific", or the lesser-used concept of "astonishing".

There are no exact equivalents. Originally I presume that FR "formidable" and EN "formidable" are cognates, just as FR "magnifique" and EN "magnificent". With time, meanings diverged.

"C'est formidable! C'est magnifique!" could be rendered as "That's marvellous! That's wonderful!" although I'm not entirely happy with that rendition, because such a repetition just doesn't seem to suit the English language as it does the French.

And I do so love Hercule Poirot, too! https://www.youtube.com/watch?v=7Vj3uS-YaHE