In a 3 gallon? I'd say 3 Guppies, max. Bigger tank is better, though. If you do go for the 3, then you have the problem of fry. If you get a male and 2 females, you will definitely have fry. If you get 3 females, you'll probably have fry. And 3 males will not work together, especially in a tank that small.
A filter for that tank could be hard...you don't want anything that will blow the Guppies around, and weaken their tails, or them. And there won't be much space to get away from the filter in that size tank....maybe try what I have in my Guppy tanks...Whisper Internal Filters. Just make sure to keep the water level almost up to the filter, otherwise if will be too strong. The only downside to this filter, is you can not adjust the flow rate. Another filter I use for my bigger Guppy tank, is the Aquaclear HOB. Then you can adjust the flow rate, to just high enough.
And yes, your friend's tank is preeeetty overstocked. And it will keep getting more and more overstocked, as long as he has Guppies, because they will keep having fry. Maybe tell him about these forums, and let people help him out?
Also, if you have a bunch of 3 gallon tanks, have you considered making one a Betta tank? Betta's are gorgeous fish, and they will absolutely love to have 3 gallons to themselves. Just be careful to not put any other fish in with one. Just to clarify...it has to be ONE Male, or ONE female. Just 1 Betta, per 3 gallon.
