pity that the tests didn´t cover the edge cases (im spoiled by other katas), e.g. granny having only 1 or no friends, or no address.
My method would fail these tests.

But it wasn't a real life scenario, so meh!

i am not sure of the other solutions, but checking if the granny has any friend or only 1 friend is mandatory!
A great programmer checks for pitfalls, even if they are not covered by tests!
'+ types for input variables
'+ type for return value

For me its easier and faster to maintain/understand without regEx.
Thats why i marked it as clever.

Even mathematicians need to code it seems.

clever, but not best practice...not maintainable by me, the least.

Looks like i am the only one that likes to explicitely declare sum as a number.

don´t fear the const.
still thumbs up for best practice.

Mathematician alert.