The link I gave is for the most likely Grant program your client got. And as you point out, it was used for expenses. That makes the income a wash against the expense; both would be reportable as pertaining to operations. It's not clear how a retail store would have this as passive income.

I have had this tab open all year:

https://commerce.mt.gov/Montana-Coronavirus-Relief/Awarded-Grants

And the link my State points us to is the same I have given you.