1988-1992 Walmart stock certificate question

I have a friend whose now deceased wife was into garage sales and estate sales. He recently found a folder in with some of her things that contains six certificates for Walmart stocks. Two of the certificates are from 1988, two are from 1990, and two are from 1992. They're each for a different number of shares. The names on the certificates are not anyone he knows since his wife is gone and he has no idea when or where she acquired them. Would it be possible for him to cash those in? We did some googling and it appears both people they belonged to are deceased. This friend of mine doesn't expect anything out of them and just showed them to us because they were cool, along with some pristine condition older stamps. He's not a wealthy guy and is retired but still working odd jobs so if they're worth anything, he could really be blessed by it. Any ideas where to get info? It has two different companies listed on them, neither of which are familiar to any of us and one has apparently been sold out twice and is now part of Bank of America. Just curious, would love to figure out if he has something big.