I don't suggest introducing this "third party". I agree with the idea about a lunch to discuss how she's doing with her work load etc. I'm sure you aren't the only one noticing that she's not pulling her weight. If other coworkers are also noticing, it will be addressed with senior management. I attend a predominantly white Irish catholic college, (DG knows what that's like
) So many other black students settle for the bare minimum in school and work. I found that talking about it is good but I also have myself to look out for. So try having lunch with her, but don't worry if she's not receptive. you probably aren't the only who feels that way.