It depends on your friends cats and the current situation. (State of roads, garden access etc)

We have four cats all adopted from no kill shelters but all found as strays. i.e. They are used to be outside.

As we live in a quiet area, with relative low traffic levels, we choose to give our cats the choice of when they are in or out (appart from late at night).

You'll find the standard for USA kitties is inside always, the possible dangers sound scary (raccoons in the garden !!)

I personally would rather our cats have a full cat life and though I worry about the risks, they seems relatively low. But if I moved to a dangerous area, I would keep them inside to keep them safe.

Unless your friends cats are in danger, I would let things be.