The best thing you can do to help your daughter in law initally is to listen to her. Embrace her and gently encourage her to speak about her feelings because most likely if she is experiencing bullying then she has only told a fraction of it.
The idea of encouraging her to talk about it is to help her realize and understand she is being bullied, and yours and other loved ones compassion can help spur her on to feel like she is regaining control. Absolutely reinforce the message there is nothing wrong with her and she is none of the things a bully has told her she is.

Empowerment is when you help another individual make the best decisions for themselves and your love and support will help empower your daugher in law so she can start developing a plan of action. If shes not in a position to find another job then her plan of action could begin with learning about what is emotional abuse, bullying and especially workplace bullying. Then she can begin finding out about the policies at work. Engage colleagues she can trust so she can learn as much as she can and create a plan she is comfortable following.

Knowledge and raising self esteem are the key issues when dealing with any kind of bullying and is even more important when its bullying in the workplace.

Do some research in your town and find out if there is a local community group who can provide support and information for workplace bullying and harrassment.

I applaud you for helping your daughter in law with this, no matter how she chooses to handle it your support will help her gain confidence and this is ultimately what she needs. I wish you all the best!