Hi Chill,
I want to confirm whether she can access the site only by her Windows Phone. Can she access the site by using computer? Which plan do you use?
Meanwhile, I suggest you check the permission of her and post the screenshot. Please follow the steps below:
1. Log into the subsite. On the left upper quarter of subsite, click Site Actions.
2. Select Site Settings. Under Users and Permissions, click Site permissions.
3. On the Ribbon, click Check Permissions, enter the appropriate user name who is able to access the subsite and click check now.
Thanks,
August Sun