Interlocutor is correct, but it is an old word that is rarely used today.
Conversationalist is someone who likes to talk. It is not correct for your sentence.
There's not a good noun for this. You might say:
- I was talking to my friend.
- I was talking with my friend.
- My friend and I were talking to each other.
- My friend and I were talking with each other.
- My friend and I were talking.
Student or Learner