Yes, you have to comply-if you do not garnish the wages, your company can be held libel for the amount owed and can sometimes be sued. There are different types of garnishments, chances are the collection agency is working for the state that is owed for student loans. There should be instructions on what to garnish(with this type of garnishment, it is typically a percentage of the pay-between 10 and 25%) and who to send the payments to. My advice is enter the garnishment, send a copy to the employee and if they dispute the garnishment, they will have to take it up with whomever is garnishing their wages. if you look at the paperwork, there might be specific laws quoted regarding the garnishment.
Garnishments can be child support, Federal or state taxes, student loans or even private debts. the paperwork you recieve for each will normally explain which it is and what is required, how much you are expected to withold etc...
basic rule of thumb: child support always comes first, the IRS can typically take as much as they want-up to 50% of gross income for a federal levy. most garnishments can take up to 25% of pay-but must typically leave the employee with a min of 30 hours at min wage(federal of $6.55) so if their gross pay is not equal to that-the full 25% won't be taken. child support in some cases can take up to 60% of pay-depending on the state and if it's in arrears. Hope this helps :)