The police would maybe look at it as assisted suicide, he took the pills and she had all the power to call for an ambulance (I know he's an evil little worm who deserves to die for killing rex, stalking Bree and being a pshychiatrists wet dream etc.), but I reckon thats they way the police will see it.
However, if they prove he killed Rex he would have been sent down anyway, and they might look at it as a life for a life.
As you pointed out she didn't force him to take the tablets, but she should have at least attempted to call for help (even though he deserved it).