If you have stopped using drugs, you do not carry your old discarded snorting straws in your pocketbook! And as it has been pointed out, there is danger that the five year old could get into momma's pocketbook. But it doesn't sound like much that he's found, just a straw, but as he put it, 'the last straw.' Apparently there's a lot of bad stuff going on here.
Something about this leads me to believe that this OP dad may not be quite as lilly white and wronged as he sounded in his first and only post, I have heard a lot of husbands and wives start raising what they hope will be really big scary issues of what their spouse is or is not doing, thinking that this will serve them well in custody and divorce. One of the biggest bar hoppers I ever knew of came in and announced that he was going to get custody of his children now, because he'd just seen his wife going into a sleazy bar. (Though of course he had no plans to change his personal habits!)
"My wife is pregnant, and she's doing cocaine!" does catch the eye, doesn't it? And my response is, if it is happening, if you honestly believe it is happening, turn her in. Her promises are worthless, and the chances of a bad outcome are quite high if she IS doing what it strongly appears she is doing...it all really deserves calling in the authorities. If it is powdered sugar he found, and she's innocent and clean as the driven snow, at least she will be motivated to think very hard about screwing up later after this has happened to her, knowing he can and will take action about it.