1) The mom has a duty/right to care for the child.
2) The child would be abandoned had the mom gone to work.
3) The phone conversation.
4) Employer's lack of other reason and the presence of evidence showing good work record.
Even if there is some sort of public policy at work here, it is overidden by the fact that the defendant could easily offer an overriding justification for the dismissal: Absences from work without the employer's permission create a burden on that employer.