How would you deal with a befriended co-worker having quit?

She hadn't told anyone that despite a temporary contract she seemed to have quit like 3 months prior. She told us of a rough phase of hers, but I told our boss that I had performed way better with her at hand, and she agreed to that. But none of us could explain why she seemed to have disappeared so suddenly. She still has her coffee box in the office, but her name is scratched out and she left the whatsapp work-group without further notice.

Update: Good news, she just told me she just quit with our present task as things do change at the end of the month. We all are just unsure about what tasks are ahead for our remaining time in contract.