some of his friends

[azz]

a. She didn't know that I had talked to some of her friends.
b. She didn't know that I had talked to certain of her friends.

If I say
c. There were some/certain of her friends that she didn't know I had talked to.

The assumption would be that she knew that I had talked to some/certain other friends of hers.

(a) and (b) could always be used if she didn't know I had talked to any of her friends, but could they be used if she knew I had talked to some/certain other friends of hers.

 

[teechar]

While some potential for ambiguity may exist, remember that context usually clarifies those things.
Note also that "certain of" is uncommon. Use "some of".