My mother's wish is to see her martyr son in a dream, what can she do?

There is nothing in Islamic law specific to seeing the deceased in a dream. However, supplication (du'a) is beneficial for every legitimate request, based on the Almighty's saying: "And your Lord says, 'Call upon Me; I will respond to you,'" and His saying: "And when My servants ask you concerning Me, then [answer them], I am indeed near. I respond to the invocation of the supplicant when he calls upon Me. So let them respond to Me [by obedience] and believe in Me that they may be [rightly] guided."