Remember that Ripnugget's change of attitude towards the alchemist does not change his attitude towards Bruthzamus or Nualia. I would refer back to the description of Ripnugget's feelings about the people below for inspiration.
I don't see Ripnugget and Bruthzamus arguing, but I don't see any of the people below the goblins falling for an "old friend is here to talk" ruse. If anything, the party has given them a heads-up(and so yes, he would alert Nualia and the others). I can see Bruthzamus and Orik coming up to assess the threat of the party from the walls, maybe.
As for the goblins and Bruthzamus telling them to be on their guard: how confusing for them, right? Their leader is new friends with these longshanks but one of the bigbads is saying that the longshanks are here to kill. I'm assuming they aren't aware of the alchemist charming Ripnugget, right? The alchemist took precautions to hide it? If not, then the other goblins are probably not fooled, and are not attacking out of fear of Ripnugget's wrath, but they know something's wrong and Bruthzamus' warning could be enogh to put them over the edge and attack.
I'm with mousmous - charming Ripnugget should have limited but not useless benefits.
Do I understand that the plan is to send Ripnugget below to lure others into an ambush in front of the building on the premise of "talking to old friends"? If so that shouldn't work. No one else is charmed and no one in the place except the other goblins has any respect for Ripnugget (Bruthazmus is cuckolding him and to the rest he's just a goblin.) They aren't falling for that lame story. Who exactly are these old friends and what are their names? Now, if the pc's give Ripnugget a really good story, like Tsuto is injured and on the mainland across the bridge. They need someone to help bring him over... For reasons opposite than the pc's might expect this might be very effective on Bruthazmus and would definitely work on Lyrie.
If the pc's use the charm to encourage Ripnugget to come outside himself for private negotiations - that seems very plausible. Not very honorable if they then ambush him but hey, murderhobos are murderhobos, right? It's also reasonable for them to get Ripnugget to share what he knows about Nualia, the layout of the level below, who's down there, etc. And I would think charm would also let the pc's persuade Ripnugget to order his goblins to step aside so the longshanks can go meet with the other longshanks below.
My approach is charm person lets you compromise the victim and what he knows but not his companions or allies. So a charmed enemy can be persuaded to let his guard down, look the other way and reveal secrets but not directly betray his allies/companions into obviously dangerous situations.