What is the ruling on one who dies by suicide while not praying? And what about giving charity on their behalf?

If the suicide victim was a Muslim who professed the two testimonies of faith, then according to the Sunnis, he does not become an infidel by abandoning prayer or by killing himself. He remains subject to the will of Allah Almighty. Therefore, it is permissible to supplicate for him for mercy and to give charity on his behalf.