Last evening, I had observed a Facebook post that was a picture of a dead opossum, with the description being that he drank whatever it was they had mixed and set out. This was posted by a coworker of a family member that works at this local business. Apparently, the opossum was not the intended target, they are intending to poison groundhogs who they claim are damaging the building's foundation. I am aware poisoning is an awful way for an animal to die, and opossums are protected. There has got to be some kind of legal/cruelty issue here?

No, this is not an approved way to remove nuisance wildlife. They would need to trap the groundhogs. Putting out liquid is just going to accidentally poison opossums, cats, and dogs.

No matter which technique is used to remove the groundhogs, they need a permit from the Illinois Department of Natural Resources. They can contact Jeff Horn, IDNR district wildlife biologist in Stephenson County for the permit (it's free) and for advice about humane ways to remove wildlife that are causing property damage. Jeff's number is (815) 273-2771.

If that does not work, and if non-target wildlife continue to be taken, then a call to a Conservation Police Officer may be in order. Their number is 309/788-0418.