How to apply for a urgent visa to visit terminally ill son

[Latifa20]

Please can someone help me on this matter as I've never had to deal with this sort of thing before. Here's the situation-

'U brother in law (my mother in laws nephew) has terminal cancer. Doctors say he may have weeks/months if lucky to live. He is currently in hospital. He is also illegal as he had his application rejected. All his family are in Bangladesh, we were hoping to send him to Bangladesh after his chemotherapy however he's too weak to go anywhere but now I would like to apply for his mother to visit him before the worse happens. We've got plenty of people in the family to sponsor her. My husband will provide all his details. We have a bedroom spare in our house and my sibling in laws all work so she can depend on us. But what is the best way to get started. It' needs to be done urgently as he's getting worse day by day. Plesse any help will be appreciated