If she doesn't just automatically put her children first, I'm not sure telling her would do any particular good.
If she's a close enough friend that you could just be honest with, you could try just having a conversation with her. You could start by saying something like, "It's kind of looking to me like you're putting Fred first; and I'm thinking that could turn into a problem for you if others think the same thing."
If you can't have that direct a conversation with her, you could try a "general conversation" about parents' responsibility to their children and about how children can be affected. If you had to you could even make up a story about someone you "know" (a fake "woman your husband works with" or "daughter of your mother's friend"), and make your point by telling some story. I know it may feel uncomfortable to lie, but if it's a way to make your point without alienating your friend, it may be better than saying nothing. You may not even have to go into a "whole, big, elaborate" story - maybe just enough to get the conversation started.
Of course, there's always the chance that what it looks like she's doing and what she, in fact, is really doing are two different things. I think you have to be open to that possibility before you decide to say anything.