The outrageous cost is for mold remediation when is normally expensive. 

It depends on the state but sometimes the only remedy for a default on a seller purchase money note is for the seller to foreclose and take back the property.

Is there anything he can do to remedy this situation?

So if you go through a title company and something gets missed at your closing you have a pretty good chance they will get it corrected without having to file a claim.If you only have an attorney's opinion and something was overlooked, you can seek remediation from E&O insurance. 

I have gone to the town building department to see if there is any record of a tank installation, removal or decommission (none), and have reviewed NJDEP data to see if there has been prior remediation on the property (none).  

And my response is usually, "Ok, because here we don't allow smoking inside because it'll cost us like $1k to remediate once you move out, which is why we make that clear upfront and in the lease."

How did you remedy it?

The home does mold remediation and new hardwood flooring and new carpet..the kitchen def needs updating.

So what remedies do you use to protect your interests?

I have no idea what a judge would expect the answer to be for that question if the meth house were fully remediated, but it is a risk if you don't disclose.So, in terms of disclosures, see what your state requires, and perhaps talk to an attorney if you have any questions about whether you do/don't need to disclose.