السلام عليكم If my father says astagfirullah instead of rabbi gfirlii, is his prayer valid? I have advised him but he doesn't listen, should I continue to pray with him leading me (we live far away from masjid so he leads me in prayer at home)

Imam Ibn Bāz said: The Sunnah for the person praying between the two prostrations is to say: “Rabbi ighfir lī, Rabbi ighfir lī” (My Lord, forgive me; my Lord, forgive me), and to repeat it. It is also legislated for him to say: “Allāhumma ighfir lī, warḥamnī, wahdinī, wajburnī, warzuqnī, wa ‘āfinī” (O Allah, forgive me, have mercy on me, guide me, mend my affairs, provide for me, and grant me well-being). If he supplicates along with seeking forgiveness for other matters, there is no harm. However, seeking forgiveness between the two prostrations is necessary, and this is considered obligatory by a group of the scholars.