Advice for a friend... irrational fear of sibling going outside

Hi guys,
I have a friend who is stumped about her 3.5 year old. The past few weeks, his behavior has changed radically. He now seems to be displaying some sensory issues and fears of things that he normally liked.

But this is one she has no idea how to handle....

Her son will not "allow" his sisters to go outside to play. Not even with the mom there or other friends there, or anything. The boy himself loves to play outside, but he has a fit if his sisters go outside. He says he is afraid they'll be hit by a car (the mom has NO idea where he has gotten this idea from).

Its to the point that he will drag his baby sister through the yard by her clothes to put her back in the house. Obviously the mom was there to stop it as soon as it started and now does not put the little one down outside anymore. But she's walking and really wants to get down and play. The mom says she holds her son in a bear hug until he calms down, but when she lets him down, he immediately goes after his sisters again.

Any advice?