Add a fair amount of bicarb soda with hot water, allowing the tupperware to soak for at least 24 hours.
You may need to repeat this depending on the amount of weed smell
I've had great success with this method, once using Mums good tupperware container as a bucket bong, and successfully removing all traces of "the odour".
Be very thorough when attempting to mess with mothers tupperware... They are extremely perceptive of shit like that
Also simply washing the tupperware with detergent and leaving it to air should help to clean a lot of the smell away. I find that sometimes immediately after washing the smell is noticeable, but when allowed to sit and dry, after a few days the smell kinda evaporates away.
You could try washing it/airing it etc like BigTrancer suggested and then if its still there you could cover up the smell by putting coffee beans in there. Failing that you could buy her some new tupperware
This thread is about harm reduction - if his mum finds out he screwed with the Tupperware, he's fucked!8(
Rinse it with white vinegar - this gets rid of most smells from plasticware in my experience
for special-k, it's definately harm reduction. motherly harm leaves pretty grievous wounds when it comes to defaced tupperware (unless you box her senseless when she presents it to you).