Need advice

I recently had a baby 8 weeks ago. The day of our gender reveal party my SO brother sends me a picture of a positive pregnany test, his GF is pregnant. They had only been together 3 months and decided they wanted to have a baby. They are both into drugs. They split up shortly after they told everyone they were expecting. She "thought" she had a miscarriage but never went to a doctor. She had no idea if she was or wasn't pregnant. So about 3months ago they got back together and she miraculously is pregnant. The whole time they were split up she was using heroin. About 3 weeks after they got back together they went to the doctos because she had t had a period since they split. Guess what? She's pregnant. She never had a miscarriage. She is due in September and hardly has a belly. She is on and off heroin and suboxone she smokes cigarettes and drinks nothing but Mountain Dew. My MIL has tried to talk to them both about the situation. But they cock and attitude with her. They say they have told the doctor that she uses but we don't believe them. At her last doctors apt she refused to pee in the cup. I hope the doctors are catching on. I want to call the doctors office and tell them about her drug use since idk if she has really told them. I'm just worried sick for the baby. How do I go about calling the doctors office. I want to remain annonomous about the whole thing.