Fledgling found

A friend of mine has found a fairly large fledgling that was just stood on his neighbours drive on the route of several cats.  He has put it in a cardboard box with a bit of material.  We don’t know the breed, it may be a pigeon.  It is unhurt and can almost fly, but there is no sign of the parent.  He has to go out tomorrow and I am away also, does anyone have any suggestions of what he should do with it?  The width of the box is 20cm.  The bird is quite alert but calm. Should he put it in his garden, which is pretty cat proof?  Any suggestions would be greatly appreciated, I told him I thought he should have left it where it was as the parents were probably close by watching, but he was concerned the cats would get it.