The Forever Gift by Donna K. Weaver

Rae Cavanaugh never expected to be a widow at twenty-six, and she has no idea how she’d have gotten through the last year if not for Dylan. He’s become her best friend. One day he’ll make some woman a wonderful husband. Sometimes, late at night when she’s feeling especially alone, Rae wishes it could be her. But she’s committed to not remarry until her children are grown. He’ll be taken by the time she’s free.
Dylan Rademaker’s been in love with Rae for months but hasn’t acted on his feelings because he’s known she’s needed time to heal. But now a guy at work is showing interest in her, and Dylan worries he’s waited too long.
Rae seems content without a social life, but a coworker convinces them both to sign up with an online dating service. Dylan jumps in with the desperate hope it will make Rae see him as husband material. But if it doesn’t work, he risks the torture of being her best friend while watching as she falls for another man.