Is a prayer valid if the congregant recites the Fatiha along with the Imam in a loud prayer, believing it to be a pillar that the Imam’s recitation does not fulfill on his behalf?

What your colleague did by reciting Al-Fatiha while being a ma'mum (follower in prayer) is correct, because its recitation is obligatory for the imam (prayer leader), the ma'mum, and the munfarid (one praying alone). It is mustahabb (recommended) for the imam to pause after Al-Fatiha so that the ma'mum can recite it, and it is mustahabb for the ma'mum to delay its recitation until after the imam. However, if one knows that the imam will only recite Al-Fatiha, or will recite a short surah (definition), and thus there won't be enough time for the ma'mum to recite it, then in such a case, it becomes obligatory for the ma'mum to recite it along with the imam.