Regarding borrowing a breast pump, both the FDA and Medela classify Pump in Styles as single user pumps. They are an ''open loop'' system (unlike the rental pumps which are a closed loop system). There are cases where milk can back up into the tubing, and into the internal breast pump diaphram, which cannot be removed and sterilized.
That being said, milk backing up into the tubes is pretty rare, and the owner of the pump would likely know if this had happened. In reality, many people borrow or buy used Pump in Styles, and simply buy new tubing, horns, and bottles, etc.
It's all based on the risk of disease transmission the used pump user is willing to bear (and the risk is very small).