Depends on the friends and the guns and your disposable income, etc etc
I got shit for buying a S&W SD9..better gun then you would expect. I've seen them down to $225. A couple of those stored in an ammo can with a few boxes of ammo and a cheap holster...and just sign them out like an arms room...
Alternatively, used Glocks that use the same mags as each other.
For long guns...a few Rem 870s or Mossbergs..even those chinese or turkish made copies. I'm not a shotgun lover though.
Some of the mid to lower end AR's...
Again, what's your plan? A cheap gun is better then no gun..if it can get thru a a few boxes of ammo, you should be alright for anti-looter use...or...if you fired it that much..perhaps something better should now be available to you.
The Ar's at least can upgraded over time, and easily.
Revolvers and pump guns for the untrained...meh...they are both more difficult to shoot well with limited training, lack capacity. are slower to reload...slower still with limited training.
I always thought the single shot H&R Handi rifles, when they were still made, had some utility in an end of the world situation...short term for younger or uninitiated shooters who could use it from protected/high ground position...and learn to shoot in a long term scenario..then upgrade to the combat gun. The basic advantage is price..you could buy a few of them in a common caliber that either you stock, or can be pilfered(if you are into the zombie scenario thing) put a mid price scope on them, and still be well under a basic AR.
If you've been to basic training, you can shoot an AR, "good enough" and could easily transition to a striker fired pistol..like the SD or Glock.
Most of the guys I know that would look to me for some sort of help, have some level of training with good combat guns, ie: Glock and M4's. They just may not own an appropriate gun or have it available to them.
But, I'm also careful with how I use the word "friend".