We had terrible issues with DD1 when it came to toilet training. It was mainly with poo rather than wee, but she definitely did the wee dance. Tbf, though, 4 years old isn't that unusual for her to be still having accidents. My DD only stopped at age 7. She needed help though, and like some children she appeared to be afraid of pooing so would hold it in for a very long time and then not get to the toilet in time.
Quite honestly, it's very likely not attention seeking, as her parents are not giving her attention when she does it. It's a strategy that is recommended but I found it to embarrassing not to be more proactive than that.
What is the DD wearing? If she's wearing leggings, less mess comes out, it goes down the leggings and your furniture is more safe.
Other things: DD to use the toilet as soon as she arrives and before and after eating. There's less chance of an accident that way. If she's doing the wee dance, her mum could say lightly, I think you need the toilet, well so do I, let's go together.
I wouldn't advise pull-ups or nappies, as it is obvious when she needs to go and taking her to the toilet like I've said does work. But I put DD1 in nappies a couple of times out of desperation and she still remembers it today, so it's a real humiliation at that age.
They could be a lot more proactive without thinking about a deterrent. Although incentive works, we did say sometimes, you if you would like an ice cream today as a treat, you need to stay dry.
But regardless, there shouldn't be any damage to your property. Another suggestion: provide her with an old chair that you don't mind getting damaged if you have one. We've done that with our DDs, at least it reduces stress.
HTH. YANBU here m, OP, but try not to blame the DD. It will be stressful for the parents, but it shouldn't be something that you need to worry about. I don't recall ever allowing DD's accidents ever damaging someone else's furniture. We watched her like a hawk when visiting friends, despite numerous accidents. (Wow, I'm glad that time is passed!!).