Why not just ask her how she feels about it? Most people don't talk about therapy outside of therapy. It's awkward and embarassing (other than at an online community about therapy like here). But that doesn't mean that it doesn't mean anything to us.

Just not rescheduling a session is one way to end therapy, but not a good way. You don't get any closure that way. I've ended therapy by just not going back several times, and have never felt good about that. A few times it was someone else, not me, who caused that last session to be missed, or failed to offer another, and I felt rejected. One time it was a therapist who didn't show up for an appointment. Another time, I quietly walked out after a confrontation, and nobody asked about rescheduling, and I was not okay and it would have meant a lot if someone had just asked. Then there was a secretary who cancelled my appointment three times in a row, and the last time she cancelled, I just gave up. I have carried a lot of hurt about those endings, and resented the people who effectively ended my therapy without consulting me.

Your daughter might not say anything if you just let her therapy slip away and don't reschedule. But she won't have the closure she needs, and you will be the person she resents. She might be ready to finish, and she might not, but either way she should have the opportunity to talk about it.