Besides, almost all airlines now are working with ground based medical assist agencies.
We certainly do, and those agencies can sometimes verify the bona thingies of anyone claiming to be a doc.
As far as the original question goes the cabin crew will obviously happily accept help but if there is any doubt in their minds about the competence of the medical professional they have to decline the assistance.
I'm guessing medical "Walts" are rare but they do exist.