There are two things to note here: the character of the devil in this novel, and the nature of the help or assistance sought.
The Master and Margarita operates with a highly nuanced image of the Devil as ultimately God's servant rather than His implacable adversary. Thus we see Woland, or Satan, standing up for the value and authenticity of faith against Berlioz the unbeliever, and more generally targeting the rotten ranks of the literary bureaucracy. The Devil in this novel is the agent of God's will in a world of sin where more direct divine intervention would be inappropriate.
It is the Master's disciple and mistress Margarita who makes the most direct use of the Devil's aid, by consenting to become a witch and the hostess of the Devil's spring celebration in return for demonic assistance. As an unfaithful married woman whose dearest wish is to spend her life with the Master, she could hardly have received direct aid from God, especially since her desire is emphatically mundane, in the literal sense, life in the world together with the one whom she loves.