I think he has not had a normal upbringing. It's essential to expose children to a lot of different scenarios, textures, visual experiences, etc. They need fine motor skills (developed with puzzles, drawing, turning book pages, etc.) and gross motor (exploration, climbing, walking, kiddie cars). They need someone to read to them, not to teach them letters and numbers, but to expose them to language and images and sequential events. They need to go on nature walks and pick up rocks and bring them home to paint them (paperweights for Grandma!). They need to pick up leaves and explore the different colors and shapes in the fall. They need to play in the snow. They need to go to children's museums with lots of tactile exhibits. They need to hear music, sing and dance. It's hard to "make" a kid use words, but they don't need to anticipate his every need so he never has to try either.
Shy kids don't make eye contact or respond. Some are reticent to try new things or overwhelmed by crowds. If this child likes to sing, they should have other types of music in the car and on the home speakers. They should learn, and play, all the finger-play songs with motions and words. Learning how to put words together in a song is a helpful step in using sentences.
I'd get him off the iPad and out of the house, whether Dad comes or not. Too much flat screen, push-button activity makes a kid think there's nothing else. And if he pushes a button and something happens, he didn't have to verbalize a thing. Why would he think talking is important? And an isolated sedentary child will think a room of toys is foreign and overwhelming.
Mom should take this child to the children's museum and the library story hour and the free puppet show on Tuesdays and whatever else her comity offers. She should take a bucket and go for a nature walk every day, bringing home interesting "finds". She should put him in the stroller and walk him around the pond and the park, eventually putting him on the swings and the small slides. And yes, there are other kids there - he's too little to be told to "play" with the others, but she can start up conversations with others kids and other moms, sit down with a shovel in the sand, and so on.
If he's picking up on her concerns and Dad's reticence, he may be less of an experimenter. She should have more fun with him and get him out in the world. It may take time for him to really interact but she can't stop. Her town probably has early intervention services, but from your description it doesn't sound like this child has had nearly enough social experiences to know what to do.