My brother has a polaroid mp3 player with a bad Li-ion battery. It's charge lasts for 8 to 14 minutes. What can we do to fix it?

Well you can't really fix the battery.
The most logical thing would be to replace the battery, if it is user-removable. If it isn't, you can probably send it to Polaroid for a battery replacement, but if it is out of warranty, it may cost some money.

Replace it. The cost for the battery and labor exceeds the cost of a new unit.

Take it in