So a friend of mine is broke... and wants to run his guitar into a Yamaha Power Amp (usually for PA) and then into a guitar cab.
He also wanted to run his guitar into a Peavey Bass Amp Head into a guitar cab.

I know BASS into a guitar cab will destroy it cause low frequencies, but since its guitar, does it have the low frequencies?

Option 1. Guitar >>> P.A. Power Amp>>> guitar cab

Option 2. Guitar >>> Bass Amp Head>>> guitar cab

Will he destroy his cab????
You can safely run a guitar through a bass amplifier and into whatever cabinet you want (as long as the power and ohms match up) without damage. You are correct that a bass through a guitar cabinet will eventually fry the speaker, but bass speakers are safe from guitars. Some people used to do that on purpose. Back in the late 60s and early 70s, a couple of guitarists liked playing their guitars through bass amps like the Ampeg SVT and Marshall Super Bass.

Running his guitar through a P.A. power amplifier won't destroy the cabinet unless there is a horrible power mismatch (i.e., 500-watt power amp into a 100-watt cabinet). What will happen is that his tone will suck beyond belief, as that power amplifier won't have any EQ controls. It will sound so sterile, you'll think he was playing through a bottle of Lysol. Get a good preamp to put in front of that power amplifier if he wants to play guitar through it.
"Maybe this world is another planet's hell?" - Aldous Huxley
Option 1 - Probably ok. Don't exceed the wattage of the cab.

Option 2 - That one is iffy. Guitar into a bass amp is always ok. But running that into a guitar cab? Depends on how much low-end audio is produced. If he was playing a bass into this combination, I'd say no. Since it's a guitar, I'd say watch the bass on the EQ - don't crank it all the way up. If the speakers start making a popping or scratching noise, turn it down.