It looks like Evelyn is gone too already.
The only part Thelyron plays in the Murder quest is telling you that Evelyn left so hastily that she even forgot her backpack in the other room.
Other than this, he only gives you some background story about Evelyn, nothing that would advance the quest any further.
And even what he tells you about her leaving is not advancing the quest but only a hint to bring you to the backpack. You'll find the key to her house (next to the clinic) in that backpack.
So ... Thelyron is actually not needed at all for the murder quest although he's kind of an important story part, which you'll find out later.