I know why...
Basically, he has sv_cheats 1 on for sandbox I presume? That enables you to do third person in your own server. And, SV_cheats 1 doesn't turn off unless you restart GMod. And Bolo he didn't try and hack or use it on the game, as he explained he was using it on his server with his friend. So I think he should be unbanned as he didn't do anything than not restarting GMod whilst having SV_Cheats 1 on. I know this as a a guy got banned before for doing the same thing.
Um, why would you even try that in the first place. Unless someone told you it made you invincible or something, well, that was stupid of you.
P.S. even if someone DID tell you, why would you do it anyways, to cheat?
P.P.S How could you have sv_cheats 1 on?
He didn't turn it on to cheat. SV_Cheats doesn't allow you to cheat on peoples server only on your own world/local game. Also as he joined after playing his own game it still had it on and the server thought he was trying to hack so technically he didn't even try to hack or anything it was just he clearly didn't know about it not turning off until you restart GMod. Also, for those of you wondering SV_Cheats is not a hack engine or anything it is an automatic part of GMod.
I will inform Mango of why this happened. Hope to see you unbanned.