but when someone is not mentally able to make decisions

The problem is "sound mind" is a subjective decision made by a qualitative review of a person (or two) that has financial interest in ensure they aren't "sound" based on a guidelines (the DSM) which was written by a industry trade association to further their members political and financial interests. Your "sound mind" argument has been used historically (and even currently) to justify chattel slavery ("blacks are savages and need to be controlled"), colonialism ("non-Europeans are lesser people and need us to parent them"), bigotry against women ("their wee little brains can't grasp complex subjects"), to the imprisonment of homosexuals in insane asylums, to the forced sterilization of rebellious women for "hysteria and anger self control mental illnesses". You are making the same old argument of tyrants everywhere "That person is engaging in a behavior I personally find icky and since no SANE person would engage in icky behavior, they must be insane" .. cue savages, women, homosexuals, ED, etc. You are basically claiming she has no agency (i.e. mentally ill) because you don't like her decisions and therefore she shouldn't be allowed to make them AND if she won't stop, should be forced. That's called slavery; you are just pretending it's not because you labelled it "for their own good". Just like Southern plantation owners after all blacks can't make decisions for themselves, everybody knows they are only 3/5's a man.