What has the other supervisor, A, done?
The case does not mention what advisor A has done. It really is advisor A's responsibility to work this out with B on behalf of the student, rather than leaving the student to try to work it out himself. If A is not fulfilling his responsibility to straighten this out, then it is a question of how much B has participated in the work that the student wants to publish.
An Uncommunicative Research Supervisor posted by andrew
Supervisors are often remiss in giving timely responses to their students posted by CarolineWhitbeck
What has the other supervisor, A, done? posted by Helpline Advisor 1
Student should go ahead and publish posted by Helpline Adviser 2
Research Supervision posted by rholland