Where did he buy LR from? What store or online website?
If you say eBay then it is probably a counterfeit copy and or one that has been used a few times before and Adobe has invalidated that serial number.
Need more info on the install and exactly when they get that error message.
If he bought an Upgrade version then he will also need a serial number from one of the previous versions of LR.
What exact version did he buy, Full or Upgrade?
He bought the full and not an upgrade version which is marketed and supplied by Adobe's appointed distributor in the country.
It is definitely a genuine and legal and unused copy and he has recourse, if necessary to get support from the distributor but being a weekend , we were hoping that we can get a solution from the community.
The error pops up after the 24 digit license key was keyed in. Strangely, if we choose trial, it also required the license to be entered.
Sounds more like a file/folder permissions problem. Like the folder the License file is supposed to be written to doesn't have Write permissions. There has been a rash of this going around with the newest version of OS X, 10.11.x.
Do a Google search for this problem and follow the instruction to clear it up. Just look at some of the other posts on this forum and you see quit a few of them.