Call your friend an idiot,
Caps are used for small bursts. They're for when your amp pushes its 'peak' wattage and requires a sudden burst of electric. Without a cap in this situation, if you have a small battery, your voltage will dip down.
If you get a cap, you would need an alternator big enough to not only support your system's RMS and charge your battery, but to support the cap as well.
In short, a cap is not a replacement for a new alt. Your alt has to be able to handle your system's RMS. If your alt can't handle your RMS, then your going to over work your alternator and drain your battery(s).
There is no 'rule of thumb' for needing an alternator. All cars are different. A shitty toyota might have a 50 amp alt while an explorer might have a 140 alt amp. Thats a big difference. what does your stock electrical system consume, and what size alt do you currently have? That will tell you how much power you have to work with.