Sorry but no i don't have experience of what you are going through, and don't take this the wrong way but i hope i never do. I have said to my mum on numerous occasions that i feel the normal roles have been reversed, i.e. i am now the mother and she is now the child, she has jokingly said 'yes mum' on a few times when i have asked her if she could do something for me (which always makes me smile).
I would start thinking along the lines of the person suffering form dementia as being child like, so yes i would start thinking of locking the cupboard under the sink (as you do for a small child), move the medicines out of the way so the person cannot get near them, anything you think a small child could get their hands on move somewhere safe incase your partner can get them. I bought one of those CD boxes to store mums medicine in and put it up high so she cannot get near it as there was a couple of occasions where i found she had taken extra painkillers, but the time had elapsed between her last lot that she was quite safe, but i always think well what if it had been one of the more powerful ones?