He doesn't need nerfs?! He just got buffed into viability. Tryndamere, Trundle, yorick could all take towers better than him and one of his core strengths was tower tanking which he has struggled to do since they killed his wither range. He also lost the damage on his ult a few months ago for bonus armor and MR which killed his objective control.
Other than Thornmail, which is great against Nasus as well, which items that are core got buffed for him? SV was nerfed recently, IBG has been nerfed since last season, TF has been worse since they made it more core to better AD scaling or AS scaling champs, Randuin's build path got gutted, FH has been sub-optimal for 2 seasons now. What items are you talking about?
All of Nasus's weaknesses are in tact. He has no mobility so Wither is essential for him to catch up or disengage. It is a single target slow which makes him a monster 1v1 but not so much in full fledged team fights. His Q buff is windowed by his ult and his shred was necessary for him to fight tanks which is all he and other juggernauts can reach. Darius, Garen and Illaoi all get free % max health shred or 5 armor shred in their kits either as a passive or on abilities without cost and those shreds stack with BC...
And ADC is supposed to be babysat. If they are stupid enough to be out alone without ward coverage and Nasus get's within range then they absolutely deserve to die. BTW any assassin, fighter or juggernaut can do this. Nasus is not special in this case.
Nasus mechanically is easy to play b/c he does not require a high amount of micro management in regards to his kit but he has one of the hardest Macro games of any champ. If he does not choose the right time to push, group and get objectives in can cost his team the game. Fiora can dive into the back line pretty easily and delete a target and GP can grab free kills from lane with his ult.
You can't ask for Wither nerfs without giving him mobility which would make him far more scary than his slow. A skill shot does not solve anything unless it can affect multiple targets and they already gutted the AS slow by half several years ago. They could scale the Q CDR on ult if that's what you are asking. Make it 25% rank 1 35% rank 2 and 50% at rank 3. But honestly with how he is performing I'd say he's doing just fine as is.
Again Nasus is a premier 1v1 duelist and if you fight him 1v1 you will die if he is stacked and itemized. He sucks in team fights and coordinated efforts as well. He is sitting at 49% win rate with 5 % play rate. So bias aside the statistics do not lie. Nasus IS now balanced. Your friend being a main is guaranteed to do well b/c he knows the champ in an d out. Use an example of someone who just picks Nasus up for freelo without understanding the champ and they will not look so easy and impressive.