"You are not any friend." sounds like "you are no friend". While "você não é um amigo qualquer" sounds more like "you are not just any friend, (you are my bestest friend)".

I don't know if this sentence works in Portuguese but it doesn't in English.

This sentence is nonsense in English. It literally is ungrammatical and meaningless.

English meaning is ambiguous, if barely grammatically tolerable ...

Right. In English usage, the sentence is very negative, more like"You are no friend at all". which in Portuguese might be "Voce e um amigo algum", which if I've got it right, is like "nenhum amigo."

Does this express a positive or negative sentiment in Portuguese?

What is wrong with qualquer amigo?

It's accepting “você não é um amigo“ :/ ?!