well still sounds a bit dramatic, but just in case you are correct, and he is trying to bump her off, then you need to stop him administering her medicines - all he has to do it giove her a few extra and she may not realise
there are gadgets that sort pills out - electrical things, with alarms that just need loading up so she knows whats shes taken and when - or just a simple pill container - you may be able to get some assistance with that from the pharmacist - maybe some varieties have days printed on?
whatever their motives, you do need to get them away from her because you will not rest and neither will she just in case something happens - and if it does you will blame him even if he hasnt done anything - so best not give him the chance
tell her a door chain will not stop someone who wants to break in - they will just kick the door open, so not to worry about that.
problem is, if you take that away from him, he may start doing stuff outdoors, at the shops, in the garden etc