a. I don't know if a friend has betrayed me.
b. I don't know if one of my friends has betrayed me.
c. I don't know if someone has betrayed me.
Could these sentences be uses if I have a specific person in mind? If I am trying to say
that I suspect a specific friend/person of betraying me?
That idea would be better expressed by
d. I don't know if a certain friend/person has betrayed me.
(In the cases of (c) we'd have a certain person and in the other two cases a certain friend.)