If you're looking for a meeting place or a good stop around ½ or halfway between Saint Helier and Lamballe, you can try Saint-Malo, France, which is almost 2 hours south of Saint Helier, or about 2 hours northeast of Lamballe.
About ⅔ or two thirds of the way, you can stop at Dinan, France. This is about 2½ hours south of Saint Helier, or about 1 hour east of Lamballe.
If you drive straight through without stopping at all, the total distance is 134 km. The distances shown above don't include local roads to get into each town, so if you're planning a long road trip, make sure you factor in traffic and road conditions, rest stop breaks, and interesting detours along the way. If you're in a rush, you can do the trip with zero stops, and your total travel time would be around 3 hours, 43 minutes.