You need to evaluate the cost of their Medicare supplement insurance versus how much your parents will have to pay out-of-pocket if they drop it.
One option for your mother
is to enroll in Medicare Part D. This offers prescription drug benefits only and the premium can be reasonable. If she does go this route, she should check with her local pharmacy to determine which carrier's plan is best for her. The pharmacist can help her make the best choice based on the medications she takes.
However, if she takes a lot of expensive medications, she may fall into what is known as the "donut hole." This is a gap where she must pay out-of-pocket until she reaches a "catastrophic" level of $4,050 total out-of-pocket spending. At that point, Medicare coverage will kick in again.
Another option is Medicare Part C, or Medicare Advantage. This is the HMO or PPO version of Medicare. Plans and their benefits, including prescription drugs, vary by state. Since your father gets his medications from the VA, perhaps he can drop his extra coverage, while your mother continues hers. As you can see, they have several options and a careful analysis is necessary.