At the end of the day OP, you think her behaviour is a sackable offence and UK Employment Law disagrees. You have raised it with HR and they have told you the facts. HR may well have discussed the issue with your colleague but you would not be given the details of that discussion because that, like your colleague's private life, is none of your business. You don't have to like your colleague. You don't have to be friends with her.
You admit she is good at her job: are you threatened by her performance? You say you are fiercely protective over your clients - maybe you should use some of that compassion towards your colleague. You have no idea why she is lying, yet you seem determined to vilify her to the point of trying to get her sacked from a job that you admit she does well! You could have just dropped your observations into conversation with her innocently, but instead you claim you were so uncomfortable and creeped out about it that you went behind her back to get her sacked instead. You chose to be a sneak rather than give her the benefit of the doubt.
It's strange that she makes things up, yes, but it doesn't give you the right to be so vindictive while claiming to have your clients' best interests at heart.
You have several choices:
- Ask her about her lies and watch her squirm so you can feel justified and self-righteous at outing her.
- Ignore the lies, be kind and courteous to her and accept that there may be many factors you're not aware of and she chooses not to share with you.
- Leave and take your spitefulness elsewhere.
To clarify - I'm not denying your colleague's behaviour is strange, or suggesting you have to like it. What I'm saying is, trying to get her sacked was nasty and sly, and trying to stir up support on an online forum after being told by your HR department that it doesn't affect her ability to do her job is beyond belief. Look at the numbers: 95% of the people on here think YABU. If you won't believe HR, believe the other 600+ people who voted.