I’m really sorry this is happening - it must be heartbreaking to hear. It is a good thing that you can relate to being on the receiving end of such behaviour, and that you recognise your DiL and GDDs innocence in the situation.
Agree with pps that you should create an appropriate moment to do so and reach out to your DiL.
Please be aware that this could turn into a very difficult process for you because, even if as a woman, you feel deeply for her I imagine you love your son and it may be very challenging to reconcile what she might confide with the child you raised and care for. My instinct is to suggest you give yourself a (small) window of time in which to think through a DN prepare yourself a bit for this difficulty, and for the possibility that you might discover things are are worse than you currently imagine.
As someone who used to live with a man who made me cry (and whose then 9 year old daughter even called him out on doing so to his face on at least one occasion) all I can say is that what the children saw was the tip of the iceberg. I was working so hard for them not to be aware of how desperate and miserable I really was, and I cried much, much more alone or in front of him than I ever did when they were present.
If you potentially see a repeating pattern here from his father’s behaviour, or think his father was also “shouty” and made you sad it would be well worth reading Lundy Bancroft’s book Why Does He Do That? which you can download on kindle/Apple Books apps. You might find it helps contextualise your experience a bit and/or that it gives you some context for what your DiL is experiencing and some tools to help. It’s a really compassionate book with some incredibly valuable insight to offer.
It may help to remember, if and when you feel torn about the situation, that what you can provide is some safety for your GC and that is likely best done by ensuring her mother feels she really can depend on your support. You can’t control what your son or your DiL do, but you can offer to be there for HER and your GDD. Do let DiL know that (if she feels able to confide) you will not minimize or dismiss her feelings, or excuse your son’s behaviour. That you want HER to be ok, either inside or outside the marriage, and that you will support her so she and your GC can live in safety.
Good luck.