Have any of the scholars said that whoever abandons prayer for three days has committed kufr, and if not, should this information be corrected immediately for my cousin, knowing that communication is difficult and a long time has passed since this statement was made?

The one who abandons prayer is in grave danger and exposed to Allah's punishment. Some scholars consider him to be a disbeliever (kafir) and outside the fold of Islam. There is no specific ruling stating that abandoning prayer for three days constitutes disbelief. The one who abandons prayer should be advised, the danger of his action explained to him, and he should be warned with the Prophet’s hadiths that caution against abandoning prayer. One must repent from speaking without knowledge.