1. by default, they can stay here as visitors for 6 months. The August date on the visa indicates that they have to enter the Canadian customs before that August date.
2. I do not recall getting stamped on my passport when I left Canada either from airports or on land. But the Chinese customs keep a very good track of people getting in and out by stamping their passports both when they leave/ enter China. Supposedly they have both "incoming stamps" and "outcoming stamps" (different shapes), combined the they indicate when you enter/leave China.