I don't think there's anything wrong with simply following someone, if the sole intention is to ensure that person is not breaking any laws. Just keep your distance and don't do anything that can be considered threatening. If someone feels threatened because they share a footpath with someone, perhaps they should not be out alone. You can't say that someone doesn't have the right to walk the same public route you're walking.

I dunno the deatils, if there was any harassment of any kind, but simply following someone with no illegitimate motive is not stalking, not close.