What if exceeds towing capacity? What would happen?

We have a 2014 Honda Civic sedan lx CVT with my uncle 3700 lbs trailer and car can still brake, maneuver, steer and handling well, there's nothing risk about exceeds that much weight, and tow hitch receiver nothing risk exceeds that weight,

my friend says is ok if exceeds car towing capacity as long drive slow and up to 100 miles

You will destroy your transaxle if the load is too big and it won't be covered under warranty.

Too risky. You could cause damage to both the transaxle and the engine. Any damage you would cause would not be coverage by your Honda warranty.

Good way to destroy the transmission.

In addition to the suspension, your engine and transmission may require additional cooling capacity and additional horsepower and stiff rear suspension to haul a bigger load as well. On a steep incline, you will notice a greater load on the engine and transmission, which is also causing greater stress on the body and frame, since on many cars, it's all one piece, and it can be rip apart if the stresses are too great. When the manufacturer provides an upper stress or load limit, there's a reason for it, and there's a safety factor built in, so it can be slightly exceeded, but not greatly exceeded.

Therefore, yes, you can pull more than RELATED towing capacity, works very well, but it's not recommended for long uphill distances on your area, due of stressful terrain to cause overheat. If you are going the distance, stay within the safety guidelines. And be safe.

The drive train may not be able to handle more than the rated capacity. You are practically doubling the vehicle's weight and the chassis may not be up to the job either with regular strength components.