Is the prayer of a man who performed tayammum to wash his uncle, then performed wudu and prayed Fajr after forgetting that he had performed tayammum, valid? And is he sinful?

If someone performed tayammum (definition) due to lack of water, then entered a village and found water and the ability to heat it, their prayer is invalid and they must repeat it, because tayammum is temporary, contingent on the removal of the excuse. There is no sin upon them for not intentionally praying in a state of ritual impurity, and they must repeat the prayer by consensus of the Muslims.