My bold.
True, but that comes from experience and a willingness to tell the client that they would be better waiting, regardless of what they want.
And we're only getting one side of the story to judge from.
Having recently dealt with a client whom was the kind that just would not be told, no matter how many valid points are made as to why something shouldn't be done, I would imagine even if he had presented them with a written caution, they would not have signed it - they ARE lawyers after all whom know they would be bound by it, and the last lawyer I did work for refused to pay me AND refused to even have a conversation about how I can "remedy" the situation that wasn't even of my making (it really wasn't, I came in behind another builder whom had screwed up, I pointed it out, and then they used THAT as a reason to screw me at the end and wouldn't even take my calls.)
I'd have the conversation with them about the screed, and if they cannot (won't) tell you what type (though I'm sure then know) then ask which company did it and contact them, which is where you might find them being quite circumspect about thier answers, (and if they again say they don't know, gently remind them the company name will be on the invoice they paid). If you did warn them verbally about it, and even though it's not in writing, that would still be enough to be taken into evidence if they decided to threaten court - or is a lawyer going to purger him/herself in front of a Judge? Unlikely.
They have more to lose in court arbitration that you do, as it's where they work and will be bound to tell the truth, and quite a lot of "I don't know" answers from a defendant doesn't look great.
You've got indemnity insurance right? Tell them to make a claim on that - the prospect of it all going legal and thr drawn out process that can take, might make them back off.
I'm not saying get all in thier face about it, but at the same time if you did warn them about the possible issues, you shouldn't have to be out of pocket and time because the client didn't listen.
I'd also be very wary about doing any more work for them.