Legal guardianship after bereavement

[Guest]

My friend died and left two children behind. Before she passed she met with a solicitor and gave legal guardianship over to her Mother. However her mother has been advised that the contract is null and void as long as the fathers are alive; they will need give consent to make her the official guardian. One parent (Father) is incarcerated. Does the will, the children's late mother, stand if not what can the grandmother do to ensure she child's wishes met.