Why does Monica feel responsible for Giles' death in the novel, A Mixture of Frailties?

Monica discovers Giles' body, but she is afraid she will be implicated in his death, so she takes his suicide letter and leaves. At the coroner's inquest, however, she learns that Giles did not die from the gas but suffocated on his own vomit, leaving her feeling as if she might have been able to save him had she acted differently.