Everglades National Park has some great tent sites close to Florida Bay at the tip of south Florida and has a lot of activities like kayaking, boat tours, wildlife hikes, ranger talks etc. But no beach.
I have not heard of any free beach camping opportunities in the Keys, and to my knowledge it is actively discouraged along with boondocking.
He might get lucky with a last minute paid tent site though. Some great places, like Long Key State Park have tent camping ($40 a night.) Big Pine Key Fishing Lodge, private campground, has tent sites but not on the beach or low in cost, further south en route to Key West.
Sorry to say, but you probably already realize cheap camping is hard to find in South Florida, whether in an RV or a tent, but especially on a beach.