I would not encourage you to reflow IC's on an iPhone logic board. The component density is vey high, the pcb is very thin and multi-layered and the components are very tiny. Applying heat without knowing what you are doing is a recipe for disaster. You could easily blow SMD passives off the board and/or cause solder ball explosions or shorts on the underfill-secured IC's or damage to surrounding IC's.
The proper way to do this is to replace defective components by applying the proper amount of heat with proper tools and experience.
Now, in regards to your friend's phone, it may be:
- A defective battery. Try replacing it with a good quality (iFixit) or known-good replacement.
- The ferrite bead on the gas gauge line. When they blow, they cause inconsistent readings of the battery condition.
- The U2/Tristar IC. It should be replaced, not reflowed.
If a replacement battery doesn't solve your problem, then you should look for a micro-soldering repair shop to do the diagnosis and repair.