The Birthday Girl by Sue Fortin

Friend: A person who is not an enemy or opponent; an ally.

As birthday girl Joanne turns forty, no one wants to celebrate her special day, or play along with her idea of a fun party – a weekend away in a cosy cottage in the woods.
But as her friends reluctantly gather round her it quickly becomes clear that there is more to Joanne's birthday weekend, because Joanne is planning to reveal a secret that one of her friends is hiding…

A beautiful cottage in the middle of the countryside sounds idyllic – until no one can hear your cries for help. And when Joanne’s party turns into a murder scene, one of the party guests must be the killer.

As secrets unravel, the rest of Jo's friends face a race against time to discover the murderer, before they are next on the killer's guest list…

Four friends. A party to die for. One killer surprise.