Happened to me recently in Belfast. I knew immediately from her body language that she did not want me to say anything so I just walked past and said nowt. Very hard as I am naturally chatty person who says hello to anyone but discretion was best this time and I got a nice pm later to say thanks and say hello any other time she is on her own.
I was going to say nothing but she looked at me and gave an acknowledging smile, which was twigged immediately by my girlfriend. So I had to say hi to avoid an even more awkward situation.
For the record I am with my girlfriend a mere 3 months so I personally wouldn't class it as long term just yet.