I got a reply from the council and apparently although I'm not a resident of the qualifying area, if I can "reasonably prove" I'm taking garden waste on her behalf, it's fine. I've got the same surname so I need to take one of her utility bills plus some ID of my own. I don't know what happens if the surnames are different.
They *may* challenge things if I make multiple trips in a van or take a trailer.
It's worth being educated on these things, apparently they only brought in this rule (about having to be a resident and prove it) because the neighbouring council did the same, so if "they won't take our residents' rubbish, we won't take theirs"... So presumably this will spread like a virus until every local authority is doing it. Tricky for people who live roughly between two or more Household Waste sites which are run by different councils. If you use the one that isn't "yours", they'll turn you away.