Is it permissible for us to contribute to paying for alcohol for a colleague who consumes it, and would we be sinful if we paid for it with him while we do not drink it ourselves?

It is not permissible for you both to assist this man by giving him money to buy alcohol, as this constitutes cooperation in sin, and a curse will afflict you both. Ibn Umar narrated that the Messenger of Allah, peace and blessings be upon him, said: "Allah has cursed alcohol, its drinker, its pourer, its seller, its buyer, its presser, its extractor, its carrier, and the one to whom it is carried."

Similarly, it is not permissible for you both to sit with him while he is drinking, due to the saying of the Messenger of Allah, peace and blessings be upon him: "Whoever believes in Allah and the Last Day should not sit at a table where alcohol is served."

There is no good in the companionship of such a person unless he repents. It is permissible for you both to abandon him if his companionship causes religious or worldly harm.