The most important thing is not to shut her out, try to stay supportive and on good terms. That way if she does start to think about ending the relationship, she won't be trapped there because of not having a support network to go back to.
It can be difficult not to tell her what to do, and to put this guy down all the time, but if you do she is more likely to take his side. You are better off saying, "you are a grown up, it's your decision, but we're still here for you."
i disagree though that the age difference is really much of a problem. I know plenty of people almost that far apart, including one couple where it is the woman who is older. I have met a lot of very very fit 42 year olds who are much more attractive and a lot more interesting than your average 25 year old guy. The problem is him being a married, serial adulterer and violent.