You just have to find the right moment to "dispose" of Avner. Here is an excerpt for JollyDoc's excellent STAP story hour on ENWorld where the PCs find a creative way to get Avner out of the picture:
"Another six days at sea, and the expedition reached the Maztican village of Renkrue. This was to be the last continental stop before they put out to open sea. Lavinia planned to take on provisions and do some trading with the villagers, and suggested that the crew of the two ships take some shore leave.
Anwar and Thrisp, realizing that they would be dealing with more Mazticans when they reached Farshore, took the opportunity to mingle with the villagers and learn what they could of the local customs and language. It was while they were speaking with one of the village elders, that a young boy came running up, out of breath and chattering rapidly in his native tongue. The elder’s face darkened and became visibly angry. He turned to Anwar and Thrisp, scowling deeply.
“Is there a problem?” Anwar asked.
“One of your people,” the elder snapped, “he try to buy young girl. Offer parents coins and beads for her! You bad people! We no trade with you! Go now!”
Anwar looked at Thrisp and the gnome nodded, “Avner.”
“Please,” Anwar said placatingly, “we apologize deeply for this insult. What is the penalty for such an offense among your people?”
The elder continued to frown. “Man must make payment to family, or else be flogged in square.”
“I see,” Anwar said, suppressing a smile. “Good sir, I promise you, this man will be brought to justice. I will bring him before you tonight. You have my word on it.”
“What’s your plan?” Thrisp asked as they walked back to the Sea Wyvern.
“The less you know, the less you incriminate yourself,” the bard smiled. “Don’t worry. I’ll handle everything.”
Anwar found Avner sulking in his cabin.
“What do you want?” the young noble sneered.
“I heard you got yourself in a bit of trouble this morning.” Anwar answered, grabbing a chair and making himself comfortable.
“What of it?” Avner snapped. “They’re just savages. They’d sell their own mothers for the right price.”
“Don’t get me wrong,” Anwar said, holding up his hands, “I understand perfectly. We’ve been at sea for weeks, and the scenery tends to get…routine, if you know what I mean. Lirith strikes me as a little spitfire, but even the spiciest soup gets cold after awhile.”
Avner stared at him skeptically. “What are you getting at?”
“Well,” Anwar continued, warming to the part, “it’s just that we’re going to be laying over here tonight, and it just so happens that I’ve come upon the location of a…hospitable establishment that caters to the tastes of a gentleman such as yourself.”
“Where is it?” Avner asked doubtfully. “I asked around myself, but everyone looked like I’d just killed their pet.”
“Subtlety, my friend,” Anwar replied. “You have to know whom to ask, and how to pose the question. Let’s just say that, if you’re still interested, I can promise you a night like you’ve never had.”
“What’s in this for you?” Avner asked, suspicion in his eyes.
“Nothing,” Anwar shrugged, “at least not now. When we reach Farshore, however, and you establish yourself in your uncle’s household, you might just remember a friend who did you a service.”
Avner smiled, and Anwar extended his hand, which the noble shook enthusiastically.
Later that night, Anwar and Avner slipped into the village under cover of darkness. The nobleman was dressed in his best finery, and clutched a bottle of wine under his cloak. Anwar led him to the only stone structure in town, built several years back by a Tyrite missionary. It was currently used as the chief’s dwelling and a communal hall, but Avner didn’t know this.
“Here?” he asked. “Isn’t this a bit…conspicuous?”
“It’s not a fixed operation,” Anwar answered. “Due to the somewhat stringent sensibilities of the villagers, these arrangements are made by appointment only when dealing with trade vessels. Don’t worry. You’re in good hands.”
They approached the building, and Anwar opened the door, motioning Avner inside. No sooner had the noble entered, than he was seized by a pair of burly warriors.
“What is the meaning of this?” he cried. “Unhand me! Do you know who I am?”
“Indeed they do,” Anwar chuckled behind him. “It seems that one option for repaying an insult as grave as yours is for the offender to serve as an indentured servant to the family for a period of one year. Consider your sentence a light one. Another option was for them to simply castrate you.”
“You can’t do this!” Avner screamed. “Don’t you think I’ll be missed? My uncle will have your head!”
“Oh, I don’t think so,” Anwar said quietly, pulling his cloak over his head, then leaning forward where only Avner could see his face in the shadows. When the nobleman peered within the hood, he saw his own reflection looking back at him. “I believe your uncle will welcome me as a beloved nephew.”
“Nooooo!!” Anwar heard Avner shriek as he turned and disappeared back into the night."