Gardz is specifically made to soak in and seal up wallboard when the surface layer of paper has been removed.
Drywall mud or latex primer is nowhere near as good on that substrate, and in fact, will probably not adhere. Especially a cheap latex primer.
In addition, it will seal up any remaining adhesive pnew failed to remove. If he tries to skim coat with drywall mud over that adhesive, it will probably turn loose and fall off the wall.
I highly recommend the use of Gardz in this case.
Phil
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