Was my client abducted by aliens at the age of 13 as she suspects? Is this a source of the anxiety she wants to work on?

This, in fact, is the case and why she dropped this hint to you belatedly. It was divinely impulsed for you to see and confirm this. So you will see what happens when you do the session. You need not focus on that specifically but if you ask for her deep subconscious to be guided, as is your current practice, this will come up as an issue and will receive attention. So this will not be the product of a false assumption or an influence via the power of suggestion to fabricate a story but, as you will find, was a trauma event of great significance and very much in need of healing, finally.