Reservoir Dogs beat me to the punch. (Eddie had his revelation while listening to K-Billy's Super Sounds of the Seventies).
It was her! She did it! Not the wife, not the cuckolded husband, not the other lover...
Everytime I hear this song on oldies radio, it holds my rapt attention right up to the shocking end. For a lyric-focused story-song, it has a particularly delicious melody. A more astute listener would infer that she's hiding something, the way she winds around those notes.
A number one hit for Vicki Lawrence in 1973.
Here's a strange video featuring the song.