How can I get her to see she is being selfish?

A bit of background for those who don’t know me well, here. We share custody of our oldest (18y/o) with his mother. He has been unwell for the past few months with anxiety and depression. He is autistic. My husband is out of town for work and, because it is a repair job, his return date is unknown. His ex phoned to tell him that she was sending our son home, alone, on a plane next Wednesday. This was not planned with us, and it was the first he heard of it. He tried to explain to her that he wasn’t home and didn’t know when he would get home, but she didn’t seem to comprehend. She insisted on this plan. I received a vague sms telling me what was happening and I in no way agree with this, as if something goes wrong, NEITHER parent are in town incase this adventure sends him into a panic-fit (which he has been experiencing lately). How can I get her to understand that she is putting her child’s health at risk because she is being selfish? (Hubby agrees with me and is hoping to get back in time so he can go to where the boy is and escort him back. If he can’t, I have to cancel my plans for next week, take my 7 year old and fly down to where the boy is.)