My friend wants to take her daily shot of heroin after taking a 10mg valium tabet, is this unsafe?

She uses intravenously about half a gram, and took a 10mg tablet of valium about two hours previous. I warned her about using two sedatives at the same time but she does not listen. 10mg is a relatively low dose of valium. Could this hurt her? And no, I don't support her habit, I just want to help her be safe.