I am staff, I've never weed whacked a rose. I have damaged other plants and various siding with gas powered string trimmers. It's not a pleasant event for homeowner or service provider. Whack a cable line and kill internet? Whoa, that gets ugly fast... Bad can happen fast with a string trimmer, other tools as well. It's a given risk homeowner or hired out.
But for roses? If it's going to be a regular issue I'd recommend some iron fencing that protect the base and crown. Roses tend to fend for themselves but power tools, string trimmers in particular are a huge potential threat. Fencing can disturb some of the plant's beauty. JMHO, fencing is an option. Personally I'd consider fencing a last resort. I'd have a talk with "staff" before investing any money forward. No need to spend money on things you don't need or trash up your garden. Make your priorities clear to those you employ.
People do listen, sometimes you have to tell them twice and I can't defend failure.
JMHO and experience. As a homeowner who's hired "professionals" to tackle projects outside my realm? It's no fun. Mistakes, estimates that go double. It's not how I do business but once you're stuck you're stuck.
Deal with the owner and explain what is "unacceptable". Demand replacements.
I would. Honest truth.
