A person when he/she avails an approved leave at a time having leave balance to his/her credit will be paid salary. On the contrary, avails approved leave without, having a balance in his/her leave a/c is considered to be 'LWP (Leave without pay)'. When either his application for leave is not approved/rejected and still not reporting to duty and/or abstain from work/duty without intimation will be treated as "Absent".
Your doubt is relevant to any of the above. When you say 'taken' leave without any leave to his credit, you have to check whether his application for leave was approved by his HoD or any authorised officer or not. If he had applied, no credit to his leave a/c. but still he was permitted such of his days will have to be treated as 'LWP' and no salary will be paid (not eligible). In case proportionate leave or credit is permitted as exceptional case due to genuine reason, to be adjusted against future accruals/credit of leave-in advance, he may be granted leave with pay(though it's not a right some departments follow this practice at their own discretion in a rare or rarest case, sometimes it's done secretly in order to help sincere & devoted employee whose contribution is appreciated by one and all, in short, for those in good books).