What is the company policy? Does the company usually pay sick leave for X amount of weeks a year?
If the company usually pays sick leave, & this is stated in the contract, then the company would be expected to pay the sick leave.
However, the company does not have to pay sick leave. The company can just not pay the employee for the sick leave & can get the employee to claim sick pay from social welfare. More recently a lot of companies do not pay sick leave, as it is too costly & small companies can't support paying someone on sick leave & paying a temp to do their job.
You can find out how long the employee is likely to be out sick for (whether a week or a few weeks) and you can decide if you want to get a temp in to cover the work.
Legally you do not have to pay sick leave. A lot of employees think they are entitled to paid sick leave from employers but they are not. I have worked in a number of hotels in accounts and I have never got paid sick leave. Sometimes if I am sick, I am 'allowed' get paid out of my annual leave, so I can pay my mortgage, but this reduces annual leave.