Teleconverter Assistance

I was approached by a parent at a baseball game I was shooting with questions about their teleconverter they just got.  First off, I shoot Sony, so I really don't know much about Nikon products and their interactions with teleconverters...this TC was a Vivatar.  She was displeased with the results she was getting form them and it not working well with the AF in her D40.  I know the camera she is using older and she is shooting sports with it, but that is beside the point and I want to see if I anyone knows if she would have more usability with a Sigma or Tamron 2x TC.  She gave me her price range and I know she can't afford the Nikon made one.  I also want to know if even a TC is a good choice for use with the D40.  I explained to the optics as to why she was having a hard time shooting what she wanted to shoot in the first place and the limitations that a TC does to the light.  Her son plays centerfield so he is already the farthest person from the camera about any standing position.

Thanks for any advice you can give me to help her out.

Rhett