Is it permissible for a worker to perform prayers in his office, either alone or in congregation with his colleagues, and what are the Sharia conditions for the validity of his prayer in this case?

Congregational prayer in the mosque is obligatory for the able-bodied man who hears the call [to prayer], unless there is an excuse. If his going to the mosque would harm his livelihood, it is permissible for him to pray at his workplace, and if he finds a congregation, he should pray with them. The prayer of an individual is valid, even if he deliberately misses the congregation, because congregational prayer is not a condition for the validity of the prayer, but whoever abandons it misses out on a great reward.