Are you sure she wasn’t a regular reader of alt.folklore.urban? Back in the day, people on that newsgroup made frequent and deliberate use of terms like pendantic, cow orker, and voracity (for veracity); they were a sort of shibboleth.
But these, including tenant for tenet, would be errors. There can be a gray area between error and alternative usage, but these are clearly on the error side of zone.
Whether you correct it or not depends on:
1) How sure you are that you are correct. I don’t know how many times on this forum I’ve been about to post something and then said to myself, “I had better check that,” only to discover what I had always thought was wrong.
2) The context. If it’s a casual conversation with an acquaintance, it’s best to let it go. If you are a copy editor, you had best correct it. You don’t want to be a dick. More often than not, inwardly wincing and just letting it pass is the right course of action.
And even if the usage is a correct one, you don’t have to like it. For instance, I can’t stand the adjective impactful, but there is nothing wrong with it, and in fact it occupies a unique semantic niche, so it’s actually useful. But I still hate it.