We ask Allaah to increase you in faith and piety and to help you to adhere to that which He has enjoined.
You should not hesitate to advise your colleagues and remind them about the prayer, rather that is what Allaah has enjoined. Allaah says (interpretation of the meaning):
“Let there arise out of you a group of people inviting to all that is good (Islam), enjoining Al-Ma‘roof (i.e. Islamic Monotheism and all that Islam orders one to do) and forbidding Al-Munkar (polytheism and disbelief and all that Islam has forbidden). And it is they who are the successful”
[Aal ‘Imraan 3:104]
“O my son! Aqim-is-Salaah (perform As-Salaah), enjoin (on people) Al-Ma‘roof (Islamic Monotheism and all that is good), and forbid (people) from Al-Munkar (i.e. disbelief in the Oneness of Allaah, polytheism of all kinds and all that is evil and bad), and bear with patience whatever befalls you. Verily, these are some of the important commandments (ordered by Allaah with no exemption)”
And the Prophet (peace and blessings of Allaah be upon him) said: “Whoever among you sees an evil action, let him change it with his hand (by taking action); if he cannot, then with his tongue (by speaking out); if he cannot then with his heart (by hating it and feeling that it is wrong), and that is the weakest of faith.” Narrated by Muslim, 49.
Not praying is a great evil, indeed it is kufr that puts one beyond the pale of Islam, according to the more correct of the two scholarly opinions, because the Prophet (peace and blessings of Allaah be upon him) said: “Between a man and kufr and shirk there stands his giving up prayer.” Narrated by Muslim, 82.
And he (peace and blessings of Allaah be upon him) said: “The covenant that stands between us and them is prayer, whoever gives it up is a kaafir.” Narrated by al-Tirmidhi, 2621; al-Nasaa’i, 463; Ibn Maajah, 1079.
You should not even entertain the notion of not praying, rather you should hate that thought as you would hate to be thrown into the fire, as the Prophet (peace and blessings of Allaah be upon him) said: “There are three things, whoever attains them has found the sweetness of faith: when Allaah and His Messenger are dearer to him than anything else; when he loves a person and only loves him for the sake of Allaah; and when he would hate to go back to kufr as he would hate to be thrown into the fire.” Narrated by al-Bukhaari, 49; Muslim, 60.
With regard to the excuse that your colleague gives about his clothes becoming impure, the answer is that he can clean the bathrooms without his clothes becoming impure, and he can also set aside pure clothes for praying in. You should note that one who truly respects the prayer and understands its importance and the seriousness of not praying would not give such an excuse.
It is essential to advise this man and others, and explain the ruling concerning one who does not pray, and the obligation to offer prayer on time, and that it is haraam to delay it until after work or before going to bed, because Allaah says (interpretation of the meaning):
“Then, there has succeeded them a posterity who have given up As-Salaah (the prayers) [i.e. made their Salaah (prayers) to be lost, either by not offering them or by not offering them perfectly or by not offering them in their proper fixed times] and have followed lusts. So they will be thrown in Hell”
Ibn Mas’ood said concerning al-ghayy (translated above as “Hell”): This is a valley in Hell which is very deep and has a foul taste.
And Allaah says (interpretation of the meaning):
“So woe unto those performers of Salaah (prayers) (hypocrites),
5. Those who delay their Salaah (prayer from their stated fixed times)”
Delaying prayer until its time is over is a major sin, and not praying at all is major kufr – we seek refuge with Allaah from that.
And Allaah knows best.
See also question no. 47425.