Looks like you are in the UK ...
1. Copyright/ DMCA Takedown Process
Copyright laws differ country to country, but in the USA the minute she wrote the article (not posted it ... actually wrote it and gave her ideas a tangible form) she owned the copyrights.
Showing proof of that -- and the timeframe -- should not be too hard. (email, draft doc. etc.)
In the US, we have something called a DMCA takedown letter. You write a letter to the infringing site's ISP documenting the proof, and the content gets taken down.
( In the UK, is there the equivalent to the US DMCA take down procedure)?
This is how we do it in the US:
2. Social Media Peer Pressure:
If you are active in social media ... don't underestimate the impact of peer pressure. We wrote about this recently ... might be a good approach , it certainly is used to good effect here.
You publicly out them and have your social community apply some peer pressure to the infringer. Here is an example:
http://info.icopyright.com/internet-piracy/fighting-online-content-theft-peer-pressure-can-work
Hope these links help.