As my father rejects the saying about this, I wanted to confirm if is it legitimate fr a father in law to eat in the same plate with daughter in law
And tell the believing women to reduce [some] of their vision and guard their private parts and not expose their adornment except that which [necessarily] appears thereof and to wrap [a portion of] their headcovers over their chests and not expose their adornment except to their husbands, their fathers, their husbands' fathers, their sons, their husbands' sons, their brothers, their brothers' sons, their sisters' sons, their women, that which their right hands possess, or those male attendants having no physical desire, or children who are not yet aware of the private aspects of women. And let them not stamp their feet to make known what they conceal of their adornment. And turn to Allah in repentance, all of you, O believers, that you might succeed. (Surat Al-Nur: 24:31)
Hence they are mehram for woman. It is allowed for her to eat with him as she would with her own father.
If you meant something else, please edit your question and elaborate further.
Update#1: To clarify a bit further about other in-laws so you don't confuse them:
Rasulullah (SAW) said: “Beware of entering upon women.” A man from the Ansar said, “O Messenger of Allah! What about Al-Hamu, or the wife’s in-law (the brother of her husband or his nephew, etc.)?” The Prophet, peace and blessings be upon him, replied: “The in-law of the wife is death itself.”
Commenting on this Hadith, Imam An-Nawawi, may Allah bless him, states:
“Al-Layth Ibn Sa`d holds that the ‘the in-law’ refers to a relative of the husband (other than his father and sons) such as his brother, nephew, and cousin, etc., with whom marriage would be permissible for her, if she were to be divorced or widowed.“ Those who are described of death are the husband’s brother, cousin, uncle, and all those who are not Mahram for the wife.
Above mentions all those in-laws who are not mentioned in the Ayat of
Surah Al-Nur 24:31