In this case, "hell" is a mild profanity. (It's not very likely, but some people might take offense.)
In any case, this means she is extremely healthy, there's not a thing wrong with her.
An example of how it might be used: Two friends are having lunch. One says, "It's too bad Mary is sick today; she was supposed to meet us for lunch." The other friend is surpirsed and says, "Mary's not sick. I just talked to her this morning, and she's healthy as hell! I bet she lied to you so she could skip our lunch and go to the beach with her boyfriend."
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