son’s friend’s worrying behaviour

My 10 yr old son recently told me that a friend of his has been smoking, a friend of mine also said her son had mentioned this friends behaviour as well. There has been rumours surrounding this certain lads behaviour for a while including that he is stealing, trouble is his mum is of the opinion that he can do no wrong and that he would never try smoking or stealing as he knows it is wrong. When I previously approached her before with my concerns I was told it was all lies. Trouble is, the smoking is now rumoured to be not just tobacco, I cant feel that I can tell his mum as I really don’t feel she would believe me but at the same time I don’t want to sit back and do nothing. I would want to know if it was my son. What do I do? Speak to her or keep quiet? One option I am considering is speaking to the school and seeing what they suggest…..