I think you have misunderstood the advice you were given. It is not true that が must be used in questions or that は cannot be. But because interrogative words like なに, だれ, etc. refer to undefined objects, it does not make sense to use the topic particle は with them. So if they are the subject of the sentence, then they must be marked with が rather than は. I believe this is all they were trying to get at.
The question can, however, be rephrased as
without any problem.