I'm so sorry about Foster.
I too don't think it was your fault, but that doesn't really help you because you will probably think up a ton of reasons why you think it is (the mind is mean like that sometimes). Hopefully though, with time, you will come to see it as a tragic accident as opposed to being your fault. I can relate though because it was down to my cat that one of my buns died and I blame myself, even though others say it's not my fault.
I hope Tiffany is ok. Can you maybe get her a friend quickly? Or maybe bond her with somebun else?
Binky free Foster.