What about Her — Emma Tharp

Six months ago, Kate Bergman's best friend left their dormroom and never came back...
Now, Kate can't shake the feeling that Willow, herdearest childhood friend, is still out there, still alive, and still needsher. But nobody is on her side--not even River McEwan, Willow's boyfriend.After all, the three of them used to be inseparable.
When a professor forces Kate andRiver to work together on a joint research project, a new challengesurfaces. Suddenly, their renewed friendship flares into something more. 

Soon, River is enticing Kate to dothings she's never dreamed of. But while she can't imagine walking away fromRiver, Kate can't stop thinking about Willow. How can she give herself to himwhen he belongs to her best friend?