Getting a leader to help you out is a sample Skill Challenge in the DMG. Obviously, it's exactly the sort of thing Skill Challenges are meant to be. Maybe someone who follows Dungeon mag or runs official module could chime in with a few more examples of 'befriending' (getting help or information or whatever from) an individual?
That's a solid ruling - treat it as a successful diplomacy check, so 1 success on the skill challenge. Not every DM will come up with that ruling, since the spell description is so vague.
I think you are missing my point a bit. It's one that has been true for skill challenges from the beginning - they can't be something that one single check could solve on its own.
If you are in a skill challenge to persuade a leader to help you, there has to be a reason the skill challenge does not already end after the first successful diplomacy check. If all you need to do to succeed at the skill challenge is befriend the leader, there is already an issue with the skill challenge.
Negotiating with a leader in the DMG is not just trying to make him your friend, it is trying to get tangible assistance from him - in the form of treasure or equivalent aid. (Something that this power specifically can't do.) The skill challenge requires not just befriending the leader (Diplomacy), but also persuading him that helping the PCs is in his best interests (Insight, History).
The fact you can spend a power to freely succeed at one aspect of the challenge doesn't mean the entire challenge is over.
This situation already exists elsewhere. The party gets to a raging river that they need to cross, which the DM has designed as a skill challenge. Turns out the Wizard can cast Fly - thus bypassing the Athletics check to swim through the river. How does the DM respond?
One option is to note that Fly just lets the Wizard succeed, and doesn't help his friends. One free success for the Wizard - everyone else still needs to swim. You could apply the same here - the leader likes the Wizard. But he still doesn't trust his comrades. "How well do you really know them? Are you sure they have our best interests at heart?" More convincing needs to be done to gain his assistance.
Another option is that the DM improvises some new obstacles. Fly provides a free success or two, but the Wizard might need to make some other check to avoid drawing notice from enemies while flying, or may have to deal with weather that makes flying difficult. Or as they fly overhead, they notice someone drowning in the river, and need to swim after them to get them. With Instant Friends, they still need to help justify the leader giving them what they want - perhaps not just with him, but with his seneschal. Perhaps they need to find a way to convince his bodyguards that he hasn't been magically influenced. Etc.
Finally, you might let the spell bypass the encounter, but introduce a different skill challenge later. Maybe the use of magic attracted the attention of wild native spirits, and they later need to fend them off. Similarly, when Instant Friends wears off, they have to deal with the fallout - maybe the leader loses influence because of his bad judgement with the PCs. Maybe rumors spread that they enchant others to do their bidding. Either one, some other issue arises because of this that they have to deal with.
In the end, sure, not every DM will come up with answers like these. But this isn't anything new. What happens if you have a wilderness skill challenge and the party teleports to their destination? Or flies over your obstacle? Or decides to just blow up your cleverly locked door? Rituals and powers have always been able to introduce the need for a DM to think on the spot with Skill Challenges, and this one power doesn't change that.
In the end, I still haven't seen any actual situations described in which this spell would be abusive.