And a result of Americans being terrible with French.

In France -ette can be added to make a word more diminutive (see kitchenette, cigarette or bralette). It wasn't initially feminine, but kind of trended that way, and only later became associated with women.

Bernard -> Bernadette
Cole -> Colette
Antoine -> Antoinette
And, one would assume, Fred -> Fredette

Except that Fredette never really took off as a female name. What did take off was Fredet, the (French) male colloquial form of Fred (or more accurately Frederick or Friedrich). Because France uses -et as a masculine ending

But while Americans had taken to importing -ette as a feminine/diminutive ending, they never really took to -et as a masculine ending (Note that we do have some -et words imported as well, are all masculine in French, even if we don't think of them as masculine in English; see alphabet, briquet, bonnet, bracelet, billet).

So when a Fredet arrived, it was respelled in the way that Americans were familiar with, replacing -et with -ette, effectively feminizing the (originally intentionally masculine) name.