I know the phrase "I know (someone) in person/personally." However, I didn't find the word "directly" used with that phrase. Also, I don't find that "in person" and "personally" fit the situation I intend.
The situation:
Note: X, Y, and Z are all females, so I'm going to refer to them individually by "her".
X told Y about Z some basic and general information as
Z's name and workplace. Now Y wants to tell her mother
that she didn't meet, see, or talk with Z. So, Y says:
"I don't know Z directly, but I know her from X."
In person means involving someone's physical presence rather than communicating by phone, e-mail..etc
Cambridge Dictionary
Personally means if you meet or know someone personally, you meet or know them in real life, rather than knowing about them or knowing their work.
Collins Dictionary
If "directly" isn't proper, I may go with "personally".
Best Answer
If you have had no direct contact with her and know about her only through the intermediary of your friends, you can refer to that fact in a number of ways, among them:
If you've only spoken with a person on the phone or communicated via email or letter: