The Temporary Wife by Heidi McLaughlin

When single dad Colby Marshall faces the unthinkable, his ex threatening to take sole custody of their son, he knows he’ll do anything to protect his family. Even if it means asking the one woman who’s always been off-limits to marry him in a last-ditch effort to prove stability: his best friend, Gianna Stapleton.

Gianna’s been Colby’s rock for years through heartbreak, bad breakups, and late-night calls that were almost something more. But she’s also fiercely independent, with no plans to settle down. Saying yes to a fake marriage wasn’t in her plans, but the moment she slips that ring on her finger, the line between friendship and forever starts to blur.
As their carefully constructed facade becomes something far too real, they’ll have to decide if the risk is worth it and if their hearts can survive falling for the one person they were never supposed to lose.