In Act V, Scene 2 of Romeo and Juliet,
Friar Laurence learns that his letter to Romeo — which was to let Romeo know that Juliet was not really dead — never reached its intended receiver. Laurence rushes to Juliet's tomb so that she will not awaken alone in her tomb.
Unfortunately, he does not get there in time. When Friar Laurence reaches the tomb in Scene 3, Romeo has slain Paris and poisoned himself. While he looks on aghast, Juliet awakes and asks for Romeo. Laurence quickly explains that their plan did not work, and that both Romeo and Paris are dead. He implores her to leave the tomb, saying that he will "dispose of thee / Among a sisterhood of holy nuns."
Overcome by sorrow, Juliet sends him away and kills herself soon afterward. Thus, a strict interpretation of the text reveals that Friar Laurence was the last person to see Juliet alive.
However, another interpretation is possible. After the Friar exits, Juliet discovers the empty cup of poison in Romeo's hand. Hoping that some of the poison still lingers on Romeo's lips to take her life as well, Juliet declares "I will kiss thy lips. / Haply some poison yet doth hang on them / To make me die with a restorative." She then kisses him and exclaims, "Thy lips are warm!"
Some have interpreted this exclamation to mean that Romeo, though poisoned, has not yet died. The poisoned Romeo, aroused by Juliet's kiss, opens his eyes to find that she is alive, but succumbs to the poison soon thereafter. Thus, Romeo becomes the last to see Juliet alive, just moments before his own tragic end.