I say, and with God's success: Attending a funeral is not considered an excuse; if the person in i'tikaf leaves for the funeral or for its prayer, even if it is obligatory for him, his i'tikaf is invalidated. If a portion of time is spent in leaving his i'tikaf, even if he was forgetful, it invalidates his obligatory i'tikaf, which is the vowed one, and he must make it up. As for the confirmed and recommended i'tikaf, it ends with his exit without an excuse. Refer to: Al-Tahbeen 1: 351, and Al-Mabsut 3: 118, and God knows best.