As the Oxford dictionary says, the word “some” can be used with singular countable nouns to refer to a person, place, thing or time that is not known or not identified.
For instance: “Some friend you are!” which probably means “you are something called a Friend, but in fact you are not one”.
So, what if we want to say this to more that one friend? Then we should use “Friends” which isn’t a singular countable nouns. It would be “Some friends you are!”. Does this convey the same meaning?
“Some X you are!”, “Some X he is!”, “Some X they turned out to be!” and similar expressions are used (to requote one of Jimic’s definitions) “when you are annoyed, to mean someone or something has disappointed you by not behaving in the way you think they should”.
X can be singular or plural, so, for example, “Some friend you are!” and “Some friends you are!” are both possible. When you say it, you emphasise the word “you”.
I do not understand how the definition you mention about “can be used with singular countable nouns…” is supposed to relate to the use of “some” in expressions like this.