Almost certainly a blister from a plant in the garden.There are a number of plants that can do this,and,if it had been cloudy it probably wouldn't of come up at all.The juice of the plant gets on the skin and when the sunlight touches it it comes up in a blister.Was she using a strimmer or tearing up weeds etc.
I have seen this when a chap(fecking idiot)was strimming without a shirt on,later that day he was covered in these blisters(he had strimmed 'Hogweed').
I am not sure what the plant is called,but,there is a lovely plant with purple flowers that will do exactly the same thing.