Time to talk to the council. Tackle it from two approaches, impact on her quality of life and environment which they'll probably be more interested in. Look up issues with hedges leading to people being forced to take their hedges out. She can argue that it's just as life impacting and turning into a similar problem. Can it affect her pipes? Someone I know's neighbour had ivy get into their sewer pipe and clog it. The carpet was a month old when the toilet began returning the family's gifts to the sewage system. There was a lot of stress.
It's compromising her view, possible property value and chance of resale (I wouldn't buy her house if invading plants were ruining my view and was going to move in to sit on the sofa with me), stress level since it's affecting quality of life. Gardening a hobby so it's costing money and endangering wildlife trying to control it.
Morecambe bay is a SSSI and SAC, selection criteria for site Morecambe Bay, EU Code UK0013027 or so an internet search tells me.
See if it counts as a risk to the native environment, wildlife and biodiversity. There are SSSIs down there in the area that it could be a threat to. Weather is getting worse and if a bit breaks off, gets swept downstream via waterways/roads then it could invade and affect the ecosystem. Costing a lot of money to try and contain. She's having difficulty as it is. I keep wanting to say hype but I mean she shouldn't play down the weed killer strength and how resistant it is to weed killer. How it's still going despite repeated uses and it's still going. Weed killer use is bad for watercourse from her applications and treating Morecambe bay and if it escapes. Also expensive to try and contain.
Put it in writing. Then if something goes wrong she can prove they were warned.
Can she ask them to dig it out? Offer them a rose or blackcurrant as a replacement.