Lee,
you need to find a way you MIL can have her fix and some time with DS without him being smokified - it may be as simple as telling your DH to take MIL for a walk in the park and text you when the smoking is over and you'll bring DS.
If you try and stop her smoking altogether you'll have a grumpy addict to deal with - and that's probably not in your best interests.
good luck!