When someone, especially a junior, is new to a project, make sure they get oriented to it. Don't just say "I'm always available if you have any questions." Because being able to take a codebase and a bunch of (even well-written, which it probably isn't) documentation and navigate it from the start without help is a skill that they don't really teach in school (at least not at MIT).

Without being oriented, they won't know what questions to ask and then after a week of slowly plodding through the code trying to piece things together, will be too embarrassed to ask.