Thank you very much for the fast reply! I found her in Settings > Reading. I deleted her there, went to the invitations page and sent an invitation.
I asked her to check her spam folder. However, it looks like the invitation is not really sent. See http://cl.ly/image/3H000X031Y11
Note that below the invitation for pieber2013 you see "geschickt an pieber2013" which is German for "sent to pieber2013". However, below the invitation for martinaornets it does not say "geschickt an martinaornets". What might be the reason for this?