While it is none of her business, at her age and the working relationship that has been established, providing a vague reason might be the lesser of evils.
I do like WLSResources answer, you could also state that you are having an abdominal procedure that has been recommended by your provider due to failed response to other treatments options. She doesn't need to know that the abdominal surgery is WLS and the failed treatments is all previous attempts to lose weight.
This way it is vague and doesn't potentially create an uncomfortable work environment if she finds out down the road it was WLS. It also eliminates the need to remember a lie.
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