To start with you would need to know that Win 7 is no longer supported by avast, some time in 2023, over two years after Microsoft ended win7 support.
I'm not sure what would be to latest version of Avast that would be supported.
What I see as possibly your biggest obstacle is the lack of internet connection.
Yes it would be possible to download an off-line installation file for Avast (if you know the latest supported version) and a location you could download the executable on another PC save to a USB. Avast only has the current version available as an off-line installation .exe file.
Now you could install that on the win7 system, but here comes the biggest issue, with no internet connection you wouldn't be able to get Virus Definition updates and you can download those from another system.
