Solved! Go to Solution.
The visible columns is tied to the OutFields collection so the easiest way is to just set the OutFields to only the ones you want. But it seems you want attributes available for the map tip that do not show up.
While I have not done it FeatureDataGrid inherits from DataGrid so I assume you can set the Columns like you have started to below. In order to do this you need to first set the AutoGererateColumns property on the FeatureDataGrid to false. Then in your Columns definition only put the ones you do want to see\
Hope that helps
-Joe