You're not lying; just not hitting him with the rotten truth about his mother. Parents fudge the truth to their young children all the time: think Easter Bunny and Santa Claus. They don't exist but parents tell their young children that they do. Comes a time and a place for the truth.
And Mattie just could have told him that his mother died of natural causes instead of hitting him with the fact that his mother overdosed on drugs. Would have been a kinder, gentler way instead of hitting him with the heavy burden that his mother was a junkie.
If Alfie was around 18 months when his Mom "OD", he probably doesn't even remember her. His grandparents would be the only substitute parents that he has really known. Think Mattie has been brainwashing him with the old opioid issue.
So over Mattie beating a dead horse about her daughter's drug problems. Maybe she should place the blame on her own daughter. No one held a gun up to her head to start down the drug route.