My wife and I will be traveling back from Caen in Normandy in June. We will be going to CDG airport and they will be going on to southern Germany? What our are train options? Could we change to a TGV outside of Paris?
Caen is the opposite side of Paris to Charles de Gaulle airport, there is no direct service. You will have to get a train from Caen to Paris St Lazare, and then get RER line A, followed by RER line B to Charles de Gaulle airport.
"Could we change to a TGV outside of Paris?" - I think you misunderstand what a TGV is a TGV (Train à Grande Vitesse) is a French High Speed Train. TGV's go all over France. Just because it is a TGV doesn't mean it will go to any particular place, but wherever it is going it will get there faster.
If your friends are going by train to southern Germany they will probably leave from Gare de l'Est in Paris, and again there is no through service. The easiest route is to take the RER Line E from Saint Lazare to Magenta station, which is very close to Gare de l'Est and Gare du Nord.