That's a fair point. The LW doesn't actually say which one it is - maybe because they don't even know themselves; they might just have heard of Kate's dad passing from Bride without any further context, connected it with her behavior during the wedding, and jumped to conclusions. If LW is a more distant friend like it seems, Bride might have wanted to keep whatever she and Kate did/talked about to herself.

But still, those are a lot of hypotheticals, and I agree with the columnist that the best thing here is to be direct and clear.