His personal account should be untouched, as someone else mentioned it is done by user, not device. The only thing I would worry about is the possibility of the district blocking personal devices in the future. I’ve never actually bothered to look if it is a setting in Google admin.
The overall scenario is something I’ve done personally. I provided my nephews with Chromebook’s and they are able to switch between accounts. The district has a log of the device but nothing of any real use beyond that.