If he said the actress ALMOST left the show, then I think it would have been Erin. Because Erin didn't leave the show, except that bit of time when she did Joanie Loves Chachi.
Linda didn't almost leave she did leave and she was only there one season.
Also in the last season, it seemed like both Joanie was used a lot less and more of a background character. I just watched an episode from the last season, and I know it's syndicated and edited, but Joanie wasn't in the episode at all. It was the one where Roger becomes principal and Fonzie Dean of Boys at George Patton High.
Chachi seemed to have been used a lot less as well.
The importance of Joanie and Chachi, even as seperate characters since they were split for the entire season, seemed very diminished in the last season.