I know this is a very basic question, but I just wanted to double check.

My friend is running an 8-speed Sora setup on her road bike. If I give her my Tiagra RD and FD, will she need to swap her cassette and chain too? Or will it be ok just to swap the derailleurs?

Thanks!

No need to change the cassette or chain (but it may be a good opportunity to if they are high milage). It might also be a good time to replace the cables if they are getting frayed and old.

She knows it won't give her another speed though, right?

Yes, she knows. Thanks!

Have fun, and look at the Park Tools page for the set up if you haven't done it before. It is easy once you get the hang of it, but it can be maddening the first time.