I'm not from avast, but I doubt they would fall within the terms of use for the free version.
So no they can't, the avast free version is for Home and non-commercial use and an NGO (Non Government Organisation) falls outside of that permitted use.
The NGO is essentially business use, the business doesn't have to be profit making, etc. charities, schools (though I believe there is a schools discount), etc. all should be using the avast Pro for business use.