My mother-in-law asked my wife to include guava leaves to her bathing water. Is this just a pamahiin or has a scientific explanation?

Guava leaves are know to be packed with antioxidants, antibacterial and anti-inflammatory agents and are considered as a natural pain relievers. Perhaps this could be the reason your MIL is suggesting the inclusion of guava leaves in your wife's bath water. I've also read that guava leaves are good for "drying out" wounds and this could also be a reason why it is often used as an ingredient in herbal baths for confinement.

Guava leaves have antimicrobial properties which heals the wound quickly. It has scientific basis not just pamahiin.