If Stocking produces another image within 15 minutes of this post that reads "I like to suck richard" then I think that would be proof enough.
1. 15 minutes is quite short to get that friend (if it is a friend) over to his house and take the photo.
2. I don't think a friend would be willing to hold up a sign that says that anyways.
It wouldn't be irrefutable proof, but it would be proof enough for me.
The only way she would be able to fake it would be for her friend to send the image online or if they are neighbors or something.