I'm so sorry for you loss. I'm sure your Westie never meant to hurt the puppy. I know from experience as my mom bred dogs when I was young and I have bred my Yorkies. I know that another female will try to play with a puppy. I make sure my other dogs are not let near new puppies. They just want to play and mother them but really only the mom know how to be gentle instinctively.
I actually had to take a puppy away from a female dog (Bull dog) when I was around 14 and the puppy had bite marks but was OK after a vet visit. The female wanted it for her own and the actual mother let her take it.
Please don't be upset with your Westie it may have wanted to mother the puppy or just play.
