Using developing time for paper density is a recipe for inconsistency and potential longer term problems. It is much better to develop the paper fully, and get the density you need through proper exposure, including burning and dodging. If the blacks in your prints are properly exposed, they will not require special shenanigans to develop. You can simply use your paper tongs to agitate the developer a bit, or lift and submerge the paper or rock the tray gently, so that the developer is not stagnant over any particular area. Make sure the developer is fresh and warm enough (20 C), and all should be well.
As far as paper is concerned, FB has its beauty. But behind glass most papers look more or less the same.
Anyway, kind gesture of you to offer to print it for him. You deserve a nod for that.