iPhone Battery Draining Too Fast

My girlfriend's iPhone 5S drains the battery at an extremely fast pace.

Why is that happened? She has a lot of apps installed. She's downloaded Battery Saver but that doesn't help much. Is the battery defective? It'll drain even if she's not using it. The battery went from 100% to 44% in a span of 4 hours. She hasn't used her phone for more than texting.

What can I do guys?