I agree with the above.
The recovery adage MYOB (mind your own business) applies.
On the one hand I think we all get a little anxious when we see our addict lying to others, and maybe we even wish others would tell us when our addict is doing something we suspect but don't know about.
But, in the end, when others have "warned" me about my addict, it didn't really help. It might have made me angry, or verified what I suspected, but what eventually helped is just me learning what my boundaries should be and enforcing them.
The only reason in the past that I have wanted verification that my addict was using again is that I wanted an explanation of crazy behavior and the hope that if I knew what was going on, somehow I could control it. There is no controlling addiction.
So, as others mentioned, your addict's mother probably suspects, but verification doesn't help. What helps is her just getting to the place where she treats him like an adult in all aspects of his life so he has the opportunity to start acting like one if he wants to.
Just my two cents. Best wishes as you work through all this.