If he currently has Windows 10 installed on the computer he does not need to purchase an install stick from Microsoft. He can make his own free using the online Microsoft Media Creation Tool. Once you have Windows 10 installed on a machine, Microsoft has record of it and it is free to reinstall on that same machine as long as you don't change the motherboard.
And in this scenario that's exactly what I would do because either there is still an active infection or the cleaning of the infections has damaged system files. First, OP should back up his data (docs, pics, videos, etc.) and then create a USB flash drive installer with the Media Creation Tool. Then he should do a fresh, clean, from scratch install of Windows 10 making sure to delete all partitions in the front end of the installation process. I can tell you from a lot of personal experience with these things that you will save yourself a lot of time and also secure peace of mind by doing a fresh install as opposed to spending hours trying to clean infections and repair system files. Virus cleaning has its place but in some cases it's a lost cause.