Rob, random port scanning has been going on since the Internet was invented and it's likely that at some point everyone's IP address will be probed for open ports. All routers in their default configuration will block those port scans so there's nothing to worry about. Also, as an added defence, most folks have a software firewall enabled on their PCs such as the Windows one.
What makes your friend think that a specific individual is specifically targeting his PC?
"...has tried a few routers to combat this problem but he says these attacks keep happening.....and has frequently re-installed his O/S but the issue keeps coming back...."
I think that proves the point I made in my first paragraph.
"* and am I on the right lines in order to help him solve the problem?"*
If your friend doesn't need file and printer sharing with other PCs within his home then by all means turn off NetBIOS.