The following was the tip given: In nominative case, use "Keiner" for masculine nouns like "Freunde."
My question is, isn't Freunde feminine since it's plural?
no
Freunde is plural. Is the plural of Freund, which means friend (masculine). I hope it helps.
The feminine version of "Freund" is "Freundin". Also, the feminine version of "Freunde" (plural of "Freund") is "Freundinnen" (plural of "Freundin").