Which format do they play? good thing about Goblins is that you can get the deck to the next level without too much cash.
From what you tell us, probably they play some kind of casual legacy, probably the best way to have fun. If you want to kick some ass or get Krenko really quick, maybe you would like to have 4 of this fuckers in your deck:
http://magiccards.info/us/en/190.html
Other goblins that you may consider are:
Siege-Gang Commander (So rape when you sneak him in with the lackey)
Goblin Matron (Goblin Tutor)
Goblin Piledriver (A motherfucking beast)
Goblin Chieftain (Makes your goblins tougher)
Goblin Warchief (Makes them cheaper)
Goblin Granade (Excellent finisher)
But believe me, Goblin Lackey gives this deck turbospeed, therefore making it a Tier A in Legacy.