There’s some confusion here, you said he’s got cat C on his licence with no codes and cat C1+E with code 107.
The only time an employers letter was needed was when HGV testing started in the 70s, you needed confirmation that you’d been driving the appropriate class of vehicle in order to get grandfather rights for the appropriate licence.
As your mate already has cat C on his licence the above paragraph is irrelevant to him now.
With cat C and cat C1+E with code 107 on his licence he can drive a cat C vehicle with a 750 kg trailer, sorry but that’s the maximum he can legally drive.
Check with DVLA by all means but you’ll find this is correct.
Sent from my mobile.