Can I reduce voltage from 3.3V to 1.8V using a 1.5V battery?
TL;DR; Your friend is telling you how to destroy a battery, potentially violently. Use a proper voltage regulator circuit instead.



Batteries are not designed to be used a voltage regulators. While it is true that in an ideal world your proposed circuit would work (after all, an ideal battery drops its rated voltage at any current), in practice things are a little different.

All the current drawn from the 1.8V supply will have to flow from the 3.3V supply, into the battery + terminal, then out of the battery - terminal. This is in effect driving a charging current into the battery. The battery is acting as a load. 

There is a reason that the batteries have a warning "Do No Recharge" printed on them, charging them can cause venting, in a potentially violent manner. Certainly a messy one.



Now you will be wondering, but what if I were to use a rechargeable battery. Initially, it would probably work ok. However the output voltage will drop as the battery charges up. 

The voltage across the battery will keep increasing until either the full 3.3V is dropped across it, or the battery is damaged by over-voltage charging.

In any case this will not give you a stable output voltage, as the cell voltage will vary with the current drawn by the load. It is really little different to using a resistor as a regulator, which doesn't work well.



The proper solution is to use a voltage regulator circuit. There are many designs. Shunt regulators using Zener diodes, Linear Regulator ICs (e.g. LM317), Transistor series voltage regulators.