Hells no. That blood sucking, no good, money grubber! I still don't know why on earth he chose to go with the crazed noble who wanted to be king of a looted and surrounded country.
Because said noble hadn't shown any intention of severing ties with Elbreth (and we'll never know now, I guess), his land had a coast, and he was simpler than the King. Aulon's not that smart - when the troubles with Taric and Terenos started, he thought they could beat them back and then the King would punish them for their actions. If only he'd thought about Elbreth's politics and Terenos' position with the King before starting all this!
But my point wasn't that I expected the King to trust Aulon. It just seems like he refuses to suspect Terenos of anything at all unless someone else (in this case, Knavston Black) he likes does it first.
The issue is how Erik perceives Aulon, actually.
Had Trubaldsome made the claim, or even written the letter himself, things would have gone differently. Favor plays a big role, but not to the extent you're thinking. Had Trubaldsome written the letter, Erik might have seen it as a paranoid (and rightly so, considering the way things had gone in Elbreth and in Miring) Prince fleeing to a place of safety after realizing the inherent treachery which you accuse Terenos and Taric of. Erik may or may not have decided that Terenos' implicit guilt was simply a mistake on Trubaldsome's part depending on how Terenos reacted after being arrested.
Instead though, Erik received a letter from a beggared nth-son of some up-jumped merchant House who was trying to make excuses after he had decided to side with a Prince that had brought nothing but trouble to court and then proceeded to cut and run from his duties in the Elbrethian army, completely ignoring the hospitality and protection Erik had shown him, in order to claim a throne atop the corpses of Erik's sworn soldiers and friends*.
On top of that, you also need to take Trubaldsome's speech into consideration. The exact words may not have traveled with word of the massacre, but the vehemence directed at Elbreth, its forces, and its King no doubt did. Trubaldsome is a traitor, plain and simple, and why should some greedy storm coast nobleman who's sided with him be trusted in even the slightest sense?
*The soldiers may or may not have also seen Erik is a "friend". I suppose it doesn't matter. The point is that Erik takes the time to know the people in his army, and so killing them is the last thing anyone would want to do if they intended to remain on his good side. This has been apparent pretty much every time he's interacted with a regiment.