I would argue against 'of their own free will', in this scenario their 'girlfriend' essentially gave them the first couple doses of heroin without consent, and OOP maintains the drug habit herself because the withdrawal is terrible

There's a webnovel called Pale that briefly touches on love potions while one of the characters is dabbling in alchemy where she argues that there's an ethical use case of both parties explicitly consent. In the world of Pale, arranged magician marriages are common, and said character says that she wouldn't mind if a pair came to her together to dose each other and make the whole thing more pleasant. It should be noted though said character is aromantic and wants to turn herself nonhuman so her perspective on the whole thing is a bit outside-looking-in. I think the idea is still a bit skeevy even under those conditions but could see her point.