If she doesn't have your old BB, then the only explanation (assuming she's being truthful about this not being your lost BB) is that your Yahoo mail account has been associated with the PIN number of her (Elizabeth's) BB. You didn't mention in your original post if you were on BES or BIS, but I assume BIS. I would, as someone suggested earlier, log into your T-Mo BIS account and make sure that the Yahoo account isn't there (if you still have access to that site).
It would actually be strange if you found the Yahoo account there and it was associated with the wrong PIN number, because you said that it was working fine with your old BB before you lost it and nothing should have changed in the meantime.
Another explanation might be some weird stray electron thing that happened when you cancelled your BB service with T-Mo. Perhaps they made some unintentional change (that I can't even guess at) when they cancelled your account.
You need a lot more info from "Elizabeth," [if that's really her name
] to solve this, I think. Who her service provider is would be a good start. If she's on T-Mo, that might make a little more sense. If she's on another provider, then that's more complicated.
Where are all the technically competent people at this hour? We need a Blackberry Expert