Just seen this. As a fan of both acts, I can confidently say there aren't any photos of them together. I've never seen any on either side, anyway.
I can only assume Kate went over, did her bit and left (on or around 16 February 1979). At that time she was still heavily promoting her first two albums and was preparing for her (only) tour beginning on 2 April.
Looking at the performance on YouTube, Benny and Björn introduce her and she is seen in the same shot as them, proving she was there at the same time, but Agnetha and Frida aren't there. I wonder if she even met them?
In more recent years, when approached about an ABBA reunion, Benny mentioned Kate in context of her 2014 live show "comeback". I think he remarked that Kate was something special. From one musician talking about another, this was quite interesting.