Standing during an obligatory prayer is one of the pillars or essential parts of the prayer, and it not permissible to omit it unless one is unable to stand because of sickness or intense fear, and the like. If a person omits it deliberately then his prayer is invalid.
Hence, sitting down during an obligatory prayer when one is fully capable of praying standing up is not permissible because they have no shar'i excuse, and therefore, the prayer sitting down is invalid.
The evidence for that is the reports narrated by al-Bukhaari (1117) and others, from ‘Imraan ibn Husayn (may Allaah be pleased with him) who said:
“I had haemorrhoids and I asked the Prophet (peace and blessings of
Allaah be upon him) about prayer. He said: ‘Pray standing up. If you
cannot, them pray sitting down, and if you cannot, then on your
side.’” See al-Sharh al-Mumti’, 3/401.
We can see that the first thing that is mentioned about prayer is standing up. If one is able to do so, then he/she must. If not, then sitting down is permitted, and if even that is too hard, then one can pray on their side.
Therefore, I would certainly advise you to mention to those who pray whilst sitting down during an obligatory prayer that if they are capable of standing, then they should do so. And Allah knows best.