Even if expenses are paid, time is also valuable (assuming your friend is a researcher). Going to a multi-day event and realizing that you are not that interested could be a less than ideal experience.
Your friend could consider just trying out Julia for an afternoon to see how it feels, eg by working through the first few chapters of the manual, then decide whether it is worth it. Having some basic (even minimal) exposure to the language would also make the workshops more interesting and easier to follow.