If you transferred the lease legally, then the assignee (new lessee) assumed all liability, including the termination fee. She would have to sue you for misrepresentation, which would be practically impossible, because every vehicle lease has a termination fee, so no one could reasonably claim that they were tricked into beleiving that no fee existed -- it's right there on the face of the contract -- plain as day!
Re countersuing, you could request sanctions on grounds that the suit is frivolous (totally without merit), or you could wait to win the suit, and the file a new complaint for "wrongful institution of civil proceedings." You can't make this suit as a counterclaim, however. You have to wait to win your defensive claim.
Hope this helps.
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