I would really love for Sam and Grace to be friends! After reading over her profile, I could maybe see Sam being someone who went out of her way to stick up for Grace after the news of her pregnancy came out? She's super protective of people and hates bullies, so she'd have absolutely stood up to anyone running their mouth about Grace. She moved away after high school, but they could have kept in touch via email/social media?
And yeah, I could see Jack knowing her from their younger years! He's the kind of kid that just seems to know everyone. She definitely sees him at the Farmer's Market with his dad, who is big and boisterous and jokes around with everyone. Jack pretends to be embarrassed but clearly loves it.