But I don't think you can use them to do what you want. Maybe you can do it by creating your own window (system specific) and use SFML only for rendering.
This solution would either reduce portability or require him to learn how to do it on all systems supported by SFML (which requires him to play with Obj-C for Mac, which I can personally attest to not being fun).
The functionality requested is very simple to implement, available on every presently supported system, and very common practice in a variety of different applications -- web browsers, video games, media players, etc. It is not a high-level feature and would not be complicated to use. I think it fits SFML nicely.