Is it possible for the fed baby to be constipated?

my friend's baby is almost 5 months old, fed and does not poop. She gives him apple juice and even after that he can't poop. You can tell he is trying hard, but all comes out is gas. When he finally poops (like once in four days) his poop looks more like puree, not liquidy fed baby poop. Is it normal? My friend is not sick and does not take medications or so. What can she do to help her son? Doctors say it is normal, but I doubt it. You can see little guy is struggling and not happy. Thank you!