There are nuances and different ways you can define what the Helm of Alignment Change does. And some of these outcomes are actually positive and not what I would consider brainwashing which is a negative consequence
It's basically the argument that kidnapping and raping a woman is ok because eventually she succumbed to Stockholm Syndrome and "fell in love" with you.
Ends don't justify means. As I said, this is a sociopathic mentality.
And I will use the example of the Dragon magazine campaign I used in about 1990-1992 which was the first time I came across redemption ( it wasn't called that in those days )arcs and similar interesting and complex ways to interpret alignment and how the D&D\Forgotten Realms reflects on it
I have mentioned this particular campaign before but this time I will go into more detail to make my point
In summary two good elven brothers were adventurers for years but one day came across a tomb and when they were exploring it they encountered a Greater Succubi who killed them but turned them into Vampires as part of what she can do
Side note, this is from Lords Of Darkness 1E, released in 1988.
A succubus' kiss no longer turns players into vampires since 2e, and I'm not aware of this ability being mentioned in any other text. The monster manual mentions only the level drain.
So the Helm ended his evil ways but it didnt brainwash him because he use to be good.
Incorrect.
The helm did brainwash him. It's how the helm works.
I don't think the text described either of them as good, let alone lawful good.
About 400 years ago Jonathon and Jeremiah Morningmist were twin high elven brothers in an adventuring party that stumbled upon ancient ruins leading to a cavern complex deep within the bowels of the land. The complex was filled with treasure. However, Jonathon, a headstrong fighter, and Jeremiah, a quick-tempered mage, soon learned their new-found wealth had a price.
The dank complex was home to a vampire, which made short work of Jeremiah and several members of the adventuring band. And as fate would have it, a succubus, who surprised the remainder of the party as it fled through the long twisting corridors, killed Jonathon.
The helm didn't reverse Jon's alignment change due to becoming a vampire, it brainwashed him with the
opposite alignment of his then Chaotic Evil alignment.
You may view this as a net benefit, but it was a brainwashing. One that was his own fault for putting on the helm he'd stolen off the people he murdered, but still a brainwashing.
But the point being the Helm of AC can be used in ways where its a legitimate and positive change. It depends on the circumstances of the person or creature?
Brainwashing is brainwashing no matter how you slice it. It's denying the fundamental rights of a person to choose who they are.
In this case, assuming the succubus forced her kiss on Jon, you could argue that it took away his freedom of choice and forced him to be evil. But without knowing what his original alignment was, forcing his alignment to change to Lawful Good doesn't seem that different. A Wish spell to restore his actual alignment would be better. I can only see an alignment change from the helm as neutral at best.
That said, you aren't talking about restoring anyone's alignment. You're talking about forcing someone to change who they are "for the better" (read: so you can "romance" them, as you put it). Using magic to force this change for that reason is clearly an evil act.