Accidental overdoses on opioid painkillers is very common these days. If she got her hands on some, it wouldn't be hard. Of course, there would likely be questions if she'd done so accidentally or on purpose, and if she had no known history of abuse of these drugs, most would assume the latter.
Though sometimes people are surprised to discover a loved one has a problem, so if she didn't actually abuse drugs, maybe she could plant some evidence to make it look like she'd been using them for a while (and so it was possibly an accident).
Do you want everyone in her family, and the police, to assume it was just an accident, or do you want there to be some question before the note is found?
Also,
most suicide victims don't leave a note, so while the presence of a suicide note that is clearly not forged may be one of the most definitive signs that a death is a suicide, there can be considerable evidence, even certainty, that a death was by suicide in the absence of a note too.