Fruit juice at 4 months?

I’m asking because my MIL was sharing that babies at 4 months can drink fruit juice…really? I only know babies who can sit by themselves can try to take purée but never heard of fruit juice. She said put fruit juice in bottle to feed, she fed her children when they were babies. I’m scared of letting her take care of my baby, if ever there’s no one to look after that day. my LO isn’t able to sit independently yet too. She even showed the recipe book, that teach mummies to blend the fruits in blender for babies, book was purchased awhile ago…