Loki knew more about Midgard and its customs than he chose to let on. He wasn't an expert, even if he would only ever admit it to himself, but he was knowledgeable. He was a trickster, a word-smith; everything he did relied upon knowledge. No good prank could be pulled off without the proper knowledge, now good lie could be told without knowing the truth behind the lie. His success in his abilities relied on practice and knowledge. Much of which could be gained by pure observation. As such he was good at observing, at reading people. Thor didn't understand how he could learn so much in such a short time, and Loki had no true desire to attempt to explain it to Thor. He had tried once, when they were both young and his reputation as a prankster and trickster was beginning to cement, but Thor didn't have the patience for it and insisted that it would never be of use to him. That all he needed to triumph in battle was his strength. Loki doubted that his view-set has changed much.
Here on Midgard so much information was on hand, able to be called up on a whim. It was no wonder that he was able to learn so much in such a short amount of time.
Now he was wishing that he wasn't so good at reading people, because he could see that Tony was killing himself. Midgardians could not hold their liquor as an Asgardian could and yet sometimes it seems like Tony was drinking like one. A disconcerting notion to be sure. So he set out to find out just what this meant. Taking the laptop Tony had given him to a secluded cafe in order to do his research. A quick search on the internet (an odd name on Loki's opinion) made him wish, just a little bit, that information wasn't so readily available. 'Excessive drinking' yielded many results, none of them good, and those results led to even worse results. In the end Loki just sat back and stared at the screen, the word alcoholism glaring back at him in harsh black and white.
Did no one else know of this? How could this have passed by everyone without notice? No, Ms. Potts knew, he could see it in her face whenever she saw him drinking. So if Potts knew then it as likely that Tony's military friend knew as well, which meant that the other Avengers would likely realize what was happening if they haven't already.
If any of them noticed Loki would wager that it would be Romanov and Barton, they both knew how to find a way to see under masks. Thor probably saw nothing wrong with Tony's drinking, thinking it was the same as Asgardian drinking. Rogers might notice, but it wasn't likely, he was too easily distracted by Tony's goading. So his noticing would come later, when it might be on the verge of affecting Tony's performance in a fight, or when he decided to look past Tony's facade.
While many places suggested talking about Tony's problem or waiting until he hit what was called rock bottom to help him Loki knew that neither of those work. At least not at the moment. He had no desire to let Tony hit rock bottom and the idea of talking about it was a tedious prospect. If he was going to help Tony it was going to have to be subtle, done in a way that Tony wouldn't notice until his drinking had been cut down.
This would require planning.