It’s not so black and white. It really depends. There needs to be communication between both parties.

It’s perfectly fine if one person says, “I can’t do this. I can’t go through with this.” Asking someone to stay by you until you die, or to carry the burden of your disability or sickness is a lot. If there’s a conversation held about it and one person wants out then it’s fine.

One person simply leaving the moment it’s announced is a bit much though, especially if they decide to come back. As someone else said, it’s called having tact.