What are you expecting to gain from telling them about this issue?
You think they've broken, not one, but two sofas and known nothing about it both times. They know they've broken at least one, and think it's ok to just leave the damage and cost to OP. OP isn't the arsehole here.
You're not out any money so presumably you don't want them to pay damages?
Oh well that's ok then eh! OP pays a particular kind of insurance to cover the cost, so let's have them break two of her chairs and her bed as well. What about the repeated inconvenience? What about the fact you don't just break people's expensive possessions twice, pretend you haven't and leave?
They're not going to be able to lose that amount of weight any time soon so they can't change this overnight. (If they have a medical issue they may not be able to change it at all)
Since when did OP say they should?
Do you want them to just stand when they visit you?
How about they come up with a solution to breaking other people's furniture repeatedly. Or do they think they're entitled to break what they like and cry "fat shaming" if anyone objects to £4k of broken possessions.
Or do you want to point out specifically that you won't have them in your house any more due to their weight so they can feel badly about it? So they can feel embarrassed and ashamed?
They aren't banned from the house. They need to stop breaking thousands of pounds of furniture and pretending they haven't. They absolutely should feel ashamed of leaving without saying anything. They broke her things. Twice.
What's your goal here?
For the person who repeatedly breaks her furniture but wails "fat shaming" if the idea they should be accountable for that, to stop breaking her furniture. And how to tackle that, when the person instantly goes into defense mode, at the "audacity" should OP try and have a conversation about the thousands of pounds of damage they've caused.
The easiest thing to do is meet them at their house or out in the community without saying ANYTHING about their weight.
And if they just stop by? What does OP do?
And I do agree that your 2k sofa might look beautiful, but if it can't handle that weight then it's a shit sofa and is made for aesthetic appeal over practicality.
Of course you do. Despite many posters already having shown that specific bariatric furniture is designed for that weight, and standard furniture is not. So if a standard sofa has withstood a much greater weight, then lovely. They aren't designed too. And are not "shit" when they break because of it.