Is the employee responsible for approving bank loan applications for other employees sinful, knowing the prohibition of usury, and having advised the employees to avoid borrowing?

Working in this entity is permissible, unless it entails falling into a Sharia-forbidden act, such as aiding in usury. What you mentioned about giving the employee a guarantee or papers by which he can borrow from the bank is a type of aiding in usury. Therefore, you must refrain from providing these papers for that purpose, or leave work in a position that requires this, unless you are compelled, in which case you remain until the necessity ceases.