Flexibility is the range of motion you have in your joints. Some say that this definition only applies when there is no outside help to move the joint, but it is not correct. If the joints a greater degree of movement, if aided by an external force, then the degree of movement defines the flexibility.
Therefore, if he wants to strengthen the muscles, and these act as the external force, then the flexibility of the joints allow a greater range of motion wereYour muscles do not exert that degree of strength.
Martial Art Training
To put it simply, if you have a high degree of flexibility in the joints, the stronger you are, the more you will be able to put that to use the flexibility in the movement that you have available. It goes without saying that the more flexibility you have in a martial art, the better.