
If the dog had a chip with your fathers name originally then the dog can be scanned and the chip will identify your father as the owner. If the woman has had another chip inserted into the dog it will have her name but the day of the chip will be after date.
The papers will presume be in the name of your Dad as if there is a request to change ownership with the KC the original owner would have to sign the paperwork.
The one thing to consider is that the woman could potentially ask for monetary compensation for looking after the dog for board , food and any other expenses so if you proceed with legal action to get the dog back potentially you could have a counter claim.
I would consider taking this further as it maybe alot of hassle and expense so be prepared for that.
Here is a solicitor specialising in laws relating to dogs so hopefully should be able to advise you
http://www.doglaw.co.uk/?gclid=CO3ulMOskq0CFegmtAodmHgrlA