No, the device does not control facial expressions
From what I assume from the movie, Isabelle is just some professional who is hired by Samantha
From the script (emphasis mine):
SAMANTHA I know you’re going through a lot, but there’s something I
want to talk to you about, okay?
Well, things have felt off with us since you went to see Catherine.
(hesitant) We haven’t been having sex. I understand that I don’t have
a body and that--
THEODORE No, no, that’s just normal. When you first
start going out it’s like the honeymoon phase and you have sex all the
time. It’s normal.
SAMANTHA (still insecure, not convinced) Oh, okay.
(beat) Well, I found something that I thought could be fun. It’s a service that provides a surrogate sexual partner for an OS/Human relationship.
THEODORE Yeah. What is it?
I was corrected in the comments that no money was involved, but when I saw the scene, I was under impression that Isabelle and Samantha rehearsed whats going to happen
So, basically Isabelle is someone who is invested in the relationship. She knows whats going to happen. One may even assume that she feels being Samantha and once I even assumed that Isabelle fallen in love with Theodore.
Tl;DR: Isabelle is in full control of her face