Sorry, can't agree.
It was her responsiblity to go into the apartment and check for hazards before letting "another adult" take her toddler inside. And the minute "Steven" told her of his suspicions about the missing candy, she should have taken the child to an ER or called 911...not gone home and pieced together a story. Thank God a friend called 911.
Since she failed to do these things...and until/unless she learns to do better...someone else needs to watch over the boy.
I agree with TXWend. At the very least an investigation needs to be performed before giving the child back.
It's possible the mother is staight up. But she either let her 3 year old go into an apartment of someone she doesn't know well (poor judgement). Or she's running in drug circles (even poorer judgement). It's more likely the latter.
They ought to test the Mom for drugs.