Keeping Japanese fluency (Asking for a friend)

So just some backstory, my friend who is completely fluent in Japanese is about to serve his mandatory military service for 2 years. He plans to pursue his university education in Japan once his military service is completed, but he is kinda scared that he’ll lose his fluency due to lack of practise and communication. From what I know, military training camps(in my country) allow for smartphones and books(though I’m not sure you’ll get much time for them) and let you return home on the weekends. Do you guys have any tips or advice for my friend?

If phones are allow, fill it with movies, audio books, e-books, podcasts and such. So they can still have input at least every day. Plus, if they have a phone, conversation partners via the internet might be possible.

I have moved from country to country; and as long as I plan and reschedule well, my languages aren’t affected too much.