Possible Water Damage.

The phone showed a full charge, and then it kept turning off 10 minutes. We then tried the rice trick. After that we plugged in with only 5% left (How when it wasn't on and the battery was out, i have no idea.) It then apparently got into a bootloop (Just going off what my mom said.)
I checked the indicators an the phone one seemed fine, but the battery one was getting diluted. Would a new battery fix the issue? If you need more information just ask. My mom just wants my brothers phone fixed asap. Thanks in advance.

Also there seems to be some stuff under the glass. like smudge.