We've been down that road. No, Instant Friends does not just deliver the effects of a single skill check, it delivers much more. Yes, winning trust or friendship - let alone both - is a valid goal for a skill challenge, and quite a lot of other potential social skill challenges wouldn't be necessary if someone was already a trusted friend. As written, this power could very easily be interpreted as 'winning' such skill challenges by itself. That's not a good idea, and it could be updated, quite easily, to avoid it.
You can argue all you want that the skill challenges it could potentially break might have been better as part of a larger skill challenge, or as a single diplomacy check, or whatever - that's fine, that's more interpretation.
I think I've laid out some pretty strong points for why such simple challenges are already problematic. I notice you haven't addressed the fact that there would be many other ways to bypass such simple challenges, nor come up with any sample challenges that the power would bypass that couldn't already be bypassed by various single skill checks.
If you really want me to believe that this power is broken, then all I'm asking is for you to address my questions. I laid out two sample challenges in line with what you were describing, along with various approaches PCs could take that would resolve it just as immediately as Instant Friends. How would you react in those other cases? Would you let a single clever Bluff check convince someone to let you into the temple archives?
Really, I think Majoru Oakheart hit on a key point. The only skill challenges this would win entirely on its own would be one that you could already win through a series of Diplomacy checks and nothing else. And, honestly, any skill challenge that consists of "roll 4-12 diplomacy checks" was already a flawed one to begin with. DMG2 outright declares that such situations are a bad choice for a Skill Challenge.
In the end, if a DM creates a Skill Challenge that consists of trying to convince someone to do something they aren't particularly opposed to anyway, that DM is already inviting PCs to easily bypass the challenge. If you are making it a full-fledged challenge, I'd expect complications, risks, consequences - all elements that could restrict the use of Instant Friends.
Now, that doesn't mean every DM will run it correctly. Some things do change from one DM to the next - but, as we've noted, that applies to skills themselves as well. NPC interaction will always be influenced by the DM, and this doesn't especially change that.