newborn problems

Asking for my sis in law her baby was born 2 weeks ago at 34 weeks weighin 4 pounds and he's home now but ever since being home he can't poop she's been back to the dr 3 times they don't do much for her  she's a new mom and very worried she has tried switching her diet oh and she breastfeeds I tried telling her maybe to just try gentle ease formula cuz my son had the same problem I stoped breastfeeding and only went to gentle ease and he was fine but anyone else have any ideas the dr literally told her it's just her baby's butt hole is too tight