Turmoil at work...need advice please?

hi.... looking for a little advice please? I started a new job late last year, so am still fairly new to the company. i work two days a week with another lady, and two other women work alternate days to us. now, the dilema i have is this.... the two ladies that work together are very good friends, and i've discovered things arent always being done the way they should.... one of the women is taking time off aat least once a month, and not declaring it to the company, and the other woman, with them being friends, is covering it up for her.What would you do? i feel bad now that i know, but don't feel i have the authority to challenge it.(our manager left a few weeks ago and hasnt been replaced yet).but if i don't make somebody aware, that makes me as bad as them, surely?so....what to do?do i mention it in a light hearted way, or go straight to an area manager with my concerns?or, do i just keep my mouth shut cos its nothing to do with me? thanks for your help in advance x