if it hadnt been for johanna exposing the lie that meg would still be alive
Yes, what did he mean there?
I'm probably wrong, but I understood him as meaning that because Joanna was exposing the truth of what was happening, Jean had to do away with Meg so that things couldn't be investigated further.
She could have easily have died of a morphine overdose, either inadvertent or intentional - she was taking so much, in unregulated amounts, that I don't know how a post-mortem could pick this up.
And while she was alive it would be so easy for Jean to present Meg as "ill" by putting her into opiate withdrawal - she'd be unwell, pale, sweaty, in pain, vomiting, unable to walk - and people would assume it was from the "brain tumour".
The young woman who spoke about being a victim of Munchausen's BP from her own mother said that her mother would emply or alter the amount in her morphine capsules - it would be easy to do similar with morphine liquid.