Cowboy of Convenience by Janice Whiteaker

Grady’s spent his whole life in Moss Creek, Montana, helping out on his family’s ranch and keeping the town he loves safe. There’s nowhere else he’d rather be.
Except for Evelyn Warwick’s bed.
The newly arrived New Yorker is a breath of fresh air in his stagnant and stale life— one he seeks out at every opportunity.
Which is how he ends up claiming to be her fiancé, hoping to run her meddling twat of an ex out of town.
But it turns out a vindictive ex is the least of Evelyn’s problems, and when Grady offers his help she makes a proposition he can’t refuse.
Now he’s pretending to be her future husband indefinitely, complete with wedding plans, shared closet space, and nights in the same bed.
And it’s not long before there’s very little fake about their arrangement.