NON EEA NATIONAL GETS RESIDENCE PERMIT THEN DOES RUNNER!

I wonder if anyone can advise as to what I need to do legally with the following query?

In May of this year we got a two year residence permit from the home office on the basis of the unmarried partner rule. My partner is a non eea national and once she got the residence permit, she decided to leave me.

Should I now advise someone as to the change of circumstance?

It very much sounds like you have been used. Apart from feeling rotten that a relationship has gone wrong, I don't think you have done anything illegal as you acted in good faith. Your partner cannot get much further in the Uk as when the visa expires she will not be in a a realtionship with you. However if she has used you as a means to an end, there is the chance that she may try her luck again on some other unsuspecting person. I honestly don't know what you should do legally but if it were me I would inform the HO. Perhaps others would care to have some input as to what should be done legally.
After all, what would you do if you met another person from outside the UK and wanted to go through the process again, would the HO believe this time it is a legitimate relationship. Not your fault1