The Sheraton is above the train station, so follow the signs that say "Paris by Train" and you'll get there.
I don't understand your comment that AF lounges allow departing guests only. There is an AF lounge specifically for arriving passengers.
I met a travel companion at the Sheraton last year and it worked out fine. I strongly recommend that you make a Plan B in advance, so if either flight is delayed past a certain time, you both know to meet at your Paris hotel instead.