I would say she is definitely in breach of the lease - in serious cases, leaseholders are in danger of forfeiting their property, and you, as the freeholders, have all the power in this situation. I'm surprised that her solicitor didn't warn her that it is a condition of the lease that there be no animals. I would get a solicitor's letter sent asap.
Peter Pedant, who posts on AB, has rental properties, so might be able to give some advice from experience, if he sees this. Don't stand for it!