Best friend drama ??

So I've got a perplexing question.

I believe my best friend is lying to me.

My boyfriend and I got clean together (yay!), and are currently in a distance situation. He lives with my mom and is regularly drug tested at home. Friend told me she's seen him using regularly since I've been gone. I know he isn't. What could she have to gain from lying to me ? I don't understand the logic. She's in her own longterm relationship