I have a 125cc bike. My cousin uses my bike more than me. He is more than 110 kg in weight. So I feel that my bike's shock absorbers will be break down soon because of my cousin. So is there any simple and cheap way to maintain the shock absorber?
As long as your cousin isn't using the bike for jumps and stunts, and is just riding it on roads this shouldn't pose a problem. As @Bob pointed out in the comment below - the spring is what takes the weight. The damper is what slows down the movement of the spring, preventing bouncing.
If the shock bottoms out under his weight, then it may damage the mounting points or the shock itself, but under normal usage the piston is capable of moving it's full range without damage.
Dirt is a killer of shocks. Dirt on the cylinder ends up scoring the cylinders and getting into the seals, causing the effectiveness of the seal to drop. Then you get leaking and eventual failure. Keep the moving portions as clean as possible. On our airplanes (which don't have any dust boots), we clean the struts with hydraulic fluid and a cotton cloth periodically.