No, she can't re enter the property unfortunately (even though the tenancy has ended). She can't change the locks either. If the tenant wont leave then she'll have to get a Court Order, it's the only way I'm afraid. If she's issued the s21 then she has started on that process.
If the s21 has been filled out correctly, especially as to the dates, then possession is mandatory once it goes before the judge.
It's a crazy situation that she can't re enter her own house but she'd be the one in Court if she tried to.
Did your friend do a full credit check on this tenant before they moved in?