It really depends on how far the situation has gone, and whether there are any hard feelings developing. I've been in the exact spot that you're in, but I had the fortune of an advance understanding between me, the SM, the director and the producer that I was on board as ASM to help train the SM. Furthermore the SM was very open and genuinely interested in doing the best possible job.
It's never fun to hear that you're not doing something right, or not doing enough, so it was still rocky and I had to tread carefully. We established "off-line" time to go over things, when we weren't in earshot of the cast or director, so that I could advise on how to handle things, and also point out some things that could have been done better. Sometimes I felt like a puppet master, signaling her to do something that we'd discussed earlier. We also cleared up that she was fine with me "taking charge" if I saw the need, as long as we maintained a consistent chain of command for the cast and director. So if the director came to me for something, it was on me to say, "sure we can do that, let's just go let Sarah know as well." Or if I noticed that something wasn't being tracked, I'd say, "Sarah, do you want me to track the movement of that prop?" As long as the two of you are open and respectful of one another, and you both have the best interests of the show in mind, you can improve the situation. And your friend will learn a lot too!