How common is it for military jets to use civilian airports?
Fighter jets may fly into local fields for airshows, to re-fuel or for needed maintenance in route etc. 

What your wife may have seen may have been related to the VIP TFR's that usually pop up in the summer due to the president (or first family) heading back to their home on weekends. 

I fly out of KDYL which is fairly close (in aviation terms) and my local FBO has been warning everyone that the Secret Service has instructed them the VIP TFR is going to be common on weekends this summer. 

You can find a map of current TFR's here.