Could be a scam, but I guess it could also be a liability for everyone if they don't recommend that and someone gets sick. At least now they can say "Hey, we warned you!" and not get sued. And you will probably need to disclose that to the new owners because is you don't you could also be sued for not disclosing a known issue. Will insurance cover it? If so, I would do it. Just to keep the sale as uncomplicated as possible. You can disclose the whole incident and the remediation so they can't haggle on price if you didn't do it.