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?
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").