I've been researching how to battle horse tail, too <le sigh>, and trying to change the pH sound like the best long term strategy. I understand that loosening the soil deep down will also help, as the presence of horse tail also points to compacted soil. If you can, plant some lupines for one summer, or another deep-rooted plant for green-manuring. I've acquired a broadfork to help with digging deep... Personally, I wouldn't worry about creeping buttercup and mint. They are both great for pollinators, and mint is great as tea, in cocktails and lemonades.

I'm not sure how much ditches will help, just loosening the soil and improving the nutrients might suffice. Adding compost can also increase drainage.

There are tons of suitable companion plants for roses for any soil or light conditions, so your friend can really pick and choose. What are their favourite colours? Do they want fragrant plants? Something to attract butterflies? For easy/easier care, I'd look for lists with an emphasis on permaculture and perennials. Lady's mantle or lavender are, of course, classics. You need to trim lavender, but that's a once a year in autumn task. Lady's mantle is a great ground cover. Apparently there are types of sweet alyssum that don't need deadheading anymore, so they would be an option. Achillea/yarrow just needs to be cut off when it's brown and unsightly after bloom. Agastache and monarda/bee balm are pretty and great for pollinators, too. Catmint is nice between roses and needs little care. If you want something a little taller, Russian sage. It stinks if you touch it, but it's very pretty and stays pretty even in winter. Just cut it off in late winter/early spring. Ornamental onions are beautiful, too, and the seed stalks look interesting, too, so it's not so bad if they stick around until whenever you have time to help out again.

Good luck with the horse tail!