Tree nuts are usually processed down the same lines as other tree nuts, not requiring extensive cleaning between uses. That's why, other than differentiating coconut (which is not a tree nut biologically, but is labeled a tree nut by the government, and which does not go down those lines), most labels only say "contains tree nuts" - you need to find tree nuts that say "in our facility we only process almonds" if you want to find tree nuts that are purely only that nut.
So, depending on levels, the 7 tree nuts may be 1 tree nut type that got contaminated by being processed on a line that does mixed nuts (vs someone else's suggestion of the line chef eating them and tasting a dish). I'm not sure released autopsy reports (which I haven't seen) would help certainty on this b/c if she was so allergic, she'd probably react whether it was a speck or a whole nut from any of the 7 types...