I say, and with God's success: His worship is invalidated even if he leaves forgetfully, and he must repeat the obligatory worship. The valid excuses for leaving the worship are: for Friday prayer, to relieve oneself, to wash if he had a wet dream and cannot wash in the mosque, the mosque collapsing, being forcibly removed by an oppressor, fearing for himself or his belongings from aggressors. However, illness, visiting the sick, attending a funeral, rescuing a drowning person or a fire victim, going out for jihad, or to bear witness are not valid excuses for leaving the worship, as leaving for these reasons invalidates his obligatory worship. Refer to: Al-Hadiyyah Al-Alaiyyah p. 185, and Al-Taalimat Al-Mardiyyah p. 185, and God knows best.