First, the story makes it quite clear that their "friendship" isn't genuine.
Given that Atlas Shrugged is explicitly formulated in Aristotelian terms (even its 3 parts are named after the 3 laws of logic, which were originally investigated by Aristotle), you could argue that their friendship is, in one sense, a "utilitarian friendship," which Aristotle viewed as the "lowest level" of friendship - it doesn't necessarily involve genuine liking, and isn't necessarily based on virtue (which both of them lacked anyway). The sole basis for their relationship is the fact that they're useful to each other in some sense.
Another defining characteristic of looters was their refusal to truly acknowledge or appreciate the ways in which people are useful to them - for example, they were completely indifferent to Hank Rearden even while they benefited from his metal. Even when they gave a banquet solely in his honor, Hank remarked to Dagney that they didn't truly care about acknowledging him or his accomplishments and they were still indifferent to his metal (much to his frustration - he hoped that the banquet at least reflected them being "gracious losers" in some sense, but it wasn't even that). They went through the motions of honoring him and holding the banquet because they had some sense that that's what you do in situations like that. This is why you can have the doublethink of having a utility-based relationship while refusing to truly acknowledge the nature of that relationship or even the fact that the other person was useful to them.
Also, the utility was the fact that they helped each other in their lack of virtue - for example, Orren Boyle helped ram through the anti-dog-eat-dog rule, which everyone knew was explicitly designed to destroy Dan Conway.
In short, their friendship was utilitarian, but they didn't truly acknowledge the other person's usefulness to them, and even this "utility" was just enabling their lack of virtue.
Given that the existence of this doublethink, it's hardly surprising that their "friendship" was also defined by some degree of treachery, back-stabbing, and mutual disloyalty. After all, that's the entire basis of their relationship to begin with. Orren Boyle is useful to Jim in the beginning of the novel primarily because he helped him stab Dan Conway in the back.
The fact that he called Boyle a "fat slob" in particular was likely due to the fact that, quite bluntly, he was a fat slob (although it is still somewhat curious that Jim acknowledges that fact). He was notoriously inept, and in spite of having steel plants that were far larger than even Hank Rearden's plants (Associated Steel had 60 open-hearth furnaces vs. Rearden's 20) he never seems to get around to actually producing much steel. (A third of his furnaces were completely idle, and the rest of them were producing 300 tons of steel per furnace per day, compared with Hank Rearden who was producing an average of 750 tons of steel per furnace). He routinely misses deadlines for orders and has nothing but excuses for why he "couldn't help it." Even when he does ship stuff, it's often of inferior quality; in at least one incident, defective steel girders his company shipped caused a building collapse, killing several workers. (Ironically, he also made steel track that was evidently far heavier than was actually required for safety and durability).
Possibly more to the point, Jim recognizes Boyle's treachery and back-stabbing - for example, he calls Boyle a "fat slob" in reference to the fact that Boyle was evidently trying to sabotage the building of the John Galt Line ("if that fat slob thinks he can...").