Then you're going to run into problems with 5E, because it's not designed around that assumption.
Well... at the same time 5e isn't either designed around the assumption that you HAVE to break the rules. You can choose to run the game with NPCs built as PCs, you just have to accept the limitations if you do so.
Yep, always was a grip I've had with 5ed. I prefer the NPC to follow the same rules as the players.
It is fine to choose so. Unfortunately in this case you can't run this particular adventure without other modifications, if you rule out mechanical modifications then they will have to be narrative modifications.
For example,
mass suggestion was a good suggestion (sic!), except that it requires the Wizard to be 11th level, while
charm person is only a 1st level spell. What is the level of this NPC in the adventure? I cannot tell from the fact that he knows
charm person, and obviously if you had to bump the villain's level by more than a couple of levels it's going to seriously alter the adventure difficulty.
Perhaps the wizard could have had a
scroll of mass suggestion instead? This might be a much more acceptable narrative change. But technically, are you willing to handwave his roll to successfully activate the scroll?
Another thing that came to my mind was for the wizard to have friends or 1st-level minions which could cast one
charm person each, but IMO this is a bigger alteration to the story.
Too bad the duration itself of charm person is 1 hour only, otherwise depending on the target party composition, you could have tried to have the mad wizard charm the party's wizard, then ask him to charm the party's druid who could charm the party's bard who could charm the party's sorcerer who could charm someone else... but technically this would not have made them friendly towards the mad wizard directly and also perhaps it does nothing to someone who is already a good friend of the caster's.
...With the concentration mechanic, it is now impossible to charm that much people/monsters into working for you.
Yes, it is as simple as that. 5e just doesn't support this specific scenario as-is.
That said, 5e rules also do not support lots of other extremely specific scenarios, while at the same time they support many other specific scenarios which previous editions did not. You can come up with a very specific idea based around the exact details of each spell after all.
It's possible to manipulate good people into doing evil things by careful social manipulation and misinformation, without magic. It just takes a lot more time and effort to set up.
Definitely, this is another option which is only a narrative change and lets the OP keep his word about not allowing an ad-hoc mechanic for this NPC.
Either way, unless there is a magic item in a published book I don't know about, some change is required.