Question

a person rides a bike with a constant velocity v with respect to ground and another biker accelerates with acceleration a with respect to ground. who can apply Newton 's 2nd law with respect to stationary observer

Answer 1

the person moving with constant velocity can apply Newton's second law of motion.
this is because Newton's second law of motion is valid to be applicable in a relative rest frame which can be obtained by an observer at rest or moving with constant velocity v.The second person is in an accelerated frame with acceleration a, and to apply Newton's second law of motion he must use a correction term known as pseudo force.
now both can apply Newton's second law of motion in their respective frames