Hey, I have a few friends whose husbands are also in merchant navy and they faced the same issues with their babies too. One of the things they do a lot though is to get on regular video calls. Most of the ships are connected to the rest of the world through virtual means, and your friend can try and fix a daily time when the baby can have a face to face chat with his father. Also, ask the father to shoot lots of videos and share, so that your friend can play them at home for the little one :)
Maybe have videos of the Dad recorded so kid can watch it and feel closer to dad. Or dad can record some audios of him singing nursery tunes or reading a story for the kid to go to bed listening to
Video calls, everyday clips of daddy at work to remind baby that daddy is out there for a purpose.. he's not absent for fun. But good luck... very tough to hear these things.
Wishing you all the best