I have seen the girl a few times over the year so have no reason to believe it's a con or anything just wanting to be prepared if the 5-0 do come calling
Hopefully all the knowledge of you this girl has is the number of an untraceable punting phone and perhaps an AW profile.
Supposing the story about the DNA is true , the girl could simply contact you herself and ask you to go down to the relevant police station to provide a DNA sample ; if you believe her ask her to do this should the police request it.
I have to admit I have some strong doubts about this and would advise caution in dealing with the girl ; suggest what I said above to her , ask her what station and the name of the officer to ask for when you get there and see how she responds.
The manner of her contacting you is a bit dubious ; why not simply wait and see if the police ask for the sample and then get in touch with you , saving you the worry of your title.It sounds like she might be lining you up for something and the fact that you have seen her a few times makes you a more suitable target ; she thinks you are a nice guy , more concerned about her than a one off punter would be , and she has had a chance to build rapport and trust in you.
Be cautious as there are a couple alarm bells with this for me , the suggestion that you are the only client she has seen for a while ie special being a big one and also meaning that you would be the only person in her life aware of what has happened to her and who she can turn to.
If you have not done so already find out all you can about her on this site.
Other members, more knowledgeable than I , have recommended approaching this in a very cautious way , which is the best route whether it turns out to be true or not.