Jade and Cathy are friends, iirc that scene when talking to the elder teacher was about a farewell gift for Cathy and then the elder teacher offered Jade she could talk with her or something like that.... Think Jade's reaction was more that she wasn't really interested in talking about gardening and knitting or sowing tips but genuinely will be missing Cathy.
About Cathy going to Bella, yeah that might have hurt, but that's because she dislikes Bella because Stephen wants Bella and ignores Jade, and if that's not the reason she dislikes Bella, it's certainly not helping. Most important is if Jade was to take action against anyone it would make alot more sense if it was against Bella.
Bella is less likely to sit at the same table for lunch as Jade and Stephen now Cathy is gone, but at what cost?