If you compare the Backup Exec 2012 Compatiblility List with the one for Backup Exec 2014 you will see that we specifically state the ESX updates in the documents as and when we add support and as such 2012 does not support that update. However BE 2014 or later does.
Would recommend at least BE 15 but preferably go to BE 16 when it goes GA (is officially released) in a few weeks.
As a side note, although not offically supported or tested, it will probably work fine. U1 added vSAN and fixed a few things, but I don't see anything that would affect you backups. U2 Again nothing really stands out. Although this would not be supported, you could run your backups while you plan your upgrade to BE 15.
If he runs into trouble with his backups before upgrading to BE 15, will you be providing him with the necessary support since you it is OK to do the backups
I did state "Although this would not be supported", but having an unsupported backup is better than nothing while he installs BE 15. I don't want anyone to have to stop data protection even while unsupported. Also you can run test restores to see if your configuration works while you install/upgrade.