Baby intolerant of anyone other than parents... What to do?

My good friends have a 6 month old that went from loving everyone to not tolerating anyone but her parents. She cries even at the sight of certain people, and it’s genuine too… You can see fear. Yet other times, those same people are just fine by her. Myself included. Sometimes she’s all smiles for me, and others she’s flat out terrified at the sight of me. Not to mention this means that whenever they try to leave her with anyone—mostly grandparents—she wails the whole time. We’re talking hours. They’re new parents, confused, and very frustrated. Any thoughts? Similar experiences? Things they can do to try and break her away from this?