If we assume that Toichi is Corbeau, then Chikage didn't know until the last moment.
The note she left screamed that Chikage was Corbeau, even though we know that she wasn't if you believe my theory. She was covering up for Toichi because they didn't want their son in even more danger.
Are you already catching on? That's right, they made a sloppy mistake if she planned this with Toichi from the start. I talked about this in Is It Chikage?
I said that you could see that she was in another part of the world so it'd be impossible.
Mistake? No, she just DIDN'T KNOW.
So when did she find out? Well, I don't know if she knew he husband was alive, but she certainly didn't know of his plan. They had to meet in person for sure, so she could check if he was legit and not an imposter who worked for Snake or something.
Therefore, we can deduce that she found out AFTER she came to Japan. Then, why did she come in the first place? She must've come before Corbeau's first heist. So, why? She certainly didn't come because Toichi requested it, it could be a trap if they couldn't check his face for a mask.
She was probably genuinely worried about her son's profession. But there's also another explanation...