Beware of opening a can of worms.
You can in theory give notice to evict on two grounds - the complaints of the freeholders, and having pets.
BUT as you say its not her dog, and I would think your tenancy says no animals to be kept at the property not no animals to visit.
If you issue a section 8, and she doesn't leave you will have to take her to court, and its not likely they will back you up as she doesn't keep a dog at the address. You would have to prove she did rather than her prove she didn't I think.
If her tenancy is not fixed term you can issue a section 21 which is 2 months notice to quit for no reason. Again if she doesn't leave then its court but ifyou do everything by the book they will give you a posession order.
If she is in a fixed tenancy then you can give this notice once you are so far into it but the eviction date has to be the date at the end of her tenancy. (you will have to check this out with some one a bit more in the know than me!)
I don't know how this affects your relationship with the freeholder, but if they havn't contacted you I wouldn't be worried.
I'm presuming it wasthe other leaseholder who told you about this - so in your situation I would write to your tenant and say that you have been informed that there is a dog in the flat and remind her that this is in breach of her lease. Send it recorded delivery so you can prove she received it.
If you have no other problems with her - she pays rent and keeps the flat nice leave it be. By writing to her if the freeholder gets on to you then you have something to show them that you have acted on the information.
You could then look into the eviction further down the line.
Be aware that evicting a tenant if they don't want to go quietly is a long and expensive process, and causes animosity with someone who has your property in their care.