IMO, I think she recognized that photo of BG as being her spouse - RA.
I think she asked him about it and he denied having been on the bridge at all that day. "That's not me Kathy; I wasn't on the bridge that day, I was only on the trail". (The photo is obviously taken on the bridge - one can see the bridge clearly).
Kathy believes him and encourages him to talk to LE because he was on the trails. RA himself stated that he came forward at the encouragement of his wife ... Had it not been for her, he wouldn't have said a darn thing about it or tipped himself in IMO. But he was caught out by her so he had to.
She knows he's talked to LE ... and life goes on swimmingly for 5 long years. So, in reality she's had 5 years to believe him as there has been zero repercussions to his chat with LE.
Than, BAM, they find themselves being questionned - RA is now a POI and her reaction is, "But you told me that you weren't on the bridge Richard". With this statement she has indicated that she had queried him about that photo - about it being him. He denied it. She believed him for 5 years. Where she's at now is another ball game.
He played her. KA was another of RA's victims ... at least for 5 long years.
And, the fact that he lied to her about being on the bridge that day, despite admitting to LE that he was on platform one of that bridge is yet just another bit of "guilty knowledge" on RAs part.
IMO.