I need to settle an argument, one way or another.
I say that an uninterruptable power supply will protect a computer from a voltage spike, my boss says otherwise.

What is your opinion ?

An uninterruptible power supply (UPS) is a device that allows a computer to keep running for at least a short time when the primary power source is lost. It also provides protection from power surges.
Source: What is uninterruptible power supply (UPS)? - Definition from WhatIs.com (http://searchdatacenter.techtarget.com/definition/uninterruptible-power-supply)

Part of the designed function is to condition the incoming supply, outputting a near perfect sinewave frequency.

along with the above, UPS equipment is like most other items in that what something does and how well it does it is a function of the cost. Some UPS equipment will not offer the same level of filtering as other more expensive models. You really need to reference the specifications of the specific item.

Depends. Generally speaking, that is what they are designed for. I have a CyberPower CP685AVR that I got from either Frys or TigerDirect and it works great. We've had all kinds of storms and power surges and everything connected to it has never been affected.