Your friends are both right, but I'll try to clarify:
100% in the FFE corresponds to about 4.07V per cell. That is less than consumer electronics which typically charge to 4.2V per cell. This is done for longevity.
However, you can do even better if you want. It is best practice to store the battery below 4V per cell. I typically only charge my FFE to 80%, as that is 3.9V per cell.
How much does it matter? Hard to say for certain, but I think it worthwhile. You can use the app to notify you when you hit 80%.
My advice: Don't leave the car at high SoC for long periods. If your daily commute allows, try to only charge to 80% or so.
Other OEMs allow you to control the SoC limit. It would be great if the FFE had that functionality.