I think this needs to allow for more variants such as "nejsi kamarád" and others
The word "no" in this context means "not any", and translated to Czech is "žádný přítel".
"Nejsi kamarád" would be in English "You are not a friend".
"Jsi nijaký přítel" doesn't work?
No, it does not. Nijaký is quite awkward on its own but, foremost, it must be nejsi because Czech requires double negatives.