Laid off after 19 years work, but signed docs way too soon with unfair compensation

So my wife (japanese) had been working for 19 years at this international company with an office in Tokyo. They had been bought out recently, and as these type of companies do after a merger, my wife was laid off.
This happened 2 weeks ago, but I was in Europe visiting fam at the time.
Her HR called her suddenly out of the blue and said that she is being laid off and that she needs to sign a document for her resignation. She said like she was pressured into signing it. They said that they wouldn't give her a 400,000 yen separation bonus payment if she didn't sign right then. She panics and quickly signs… The terms are that she would be an employee for 2 months butdoesnt have to work, so basically 2 months compensation. Her 退職金 comes out of her 401k which the company had been paying half of. (Is that normal? 😩)
She signed it before she even messaged me that she had been layed off. I immediately replied "don't sign anything and don't agree to anything yet !!", but it was too late. 🙄
Is there any way that we could get this signed document retracted or get more better conditions because it's pretty dumb that she only gets 2 months worth of payment after almost 20 years. It should be more like 12 months. I don't mind doing lawyer stuff if necessary, but I did want to ask for others for real world experiences as well.
Does anyone else have experienced with this sort of thing?
Or do we just have to eat this millions of yen mistake? 😊
Thanks.

by OmaeWaUrusai