First of all, that is very very sad...
I don't co-sleep, kiddo was in a side-crib after the first three months (crazy sleeper) but I know that there are very strict rules (no covers no pillows... etc) just reassure your mother that you are taking all the precautions and the benefits of co-sleeping are immeasurable... plus a ton of people have co-slept for years and there are very very few problems when done correctly.