This is a really good point. If you do feel like you need to tell her, her first session back is probably the wrong time.

When my former marriage counselor's wife passed away, I found out not long after it had happened (she'd been sick, and he suddenly had to cancel, so I googled her and found her obituary). We let him know we'd found out, and he said he had planned not to tell us. I reacted poorly to that and was pushing him to explain why, things like that. He handled it kindly and compassionately, but I imagine not all T's would (even if under non-grief circumstances, they'd handle it well).

I know this is different, but I do think it's riskier telling her when she's likely grieving. She may react differently than if you told her you googled her when something like that wasn't going on in her life. So I'd at least wait.