TMZ has a video of Tim McGraw in concert and it appears this woman in the audience made contact with Tim McGraw's privates and he slapped her in the face.
From TMZ:
Look closely 44 seconds into the clip.  You see a woman slap Tim twice  ... first on his knee and the she goes in a second time and either comes close to his junk or makes contact with it.

At 50 seconds, Tim smacks her. 

Tim's rep tells TMZ, "Tim was singing out in the audience and someone firmly grabbed onto his leg and wouldn't let go as he was moving through the crowd.  

The rep says, "He instinctively swatted to try to keep them from ripping his jeans (which they succeeded at doing!), and so he could get to more fans who could slap hands with him before the end of the show."

Read more: