I realise this answer is probably too late for the original poster, but for others in similar situations, I would suggest looking towards 'twisty' TV shows (Think: Revenge, Twisted, Gossip Girl, etc) as inspiration.
In this case the NPC is playing the character of the bad guy/girl who cleaned up their act. In the process, they've redeemed themselves in the eyes of the protagonists (the PCs) and become a friend to some extent, and more to the point, someone the protagonists trust, confide in and rely on to some extent to take care of minor issues/problems.
Thus ends the first season, and the NPC comes in as a breakout regular character in the second season, but someone from their dark past comes back into town to elevate the drama.
In the OP's particular situation, an old friend arrives on the NPCs doorstep. Many of these situations have been alluded to already in other answers, but I think they actually all work especially well in combination. The old friend is a corrupting influence on the NPC, and the NPC isn't up to dealing with the old friend appropriately for a variety of reasons
- "Old bonds": The NPC and the old friend go way back and the NPC can't let the old friend down. The NPC genuinely wants to help, but keeps getting into trouble when they do so
- "Moral Obligation": the old friend helped out, helped raise, or in some other way did something the NPC feels they will forever be in debt for. Nothing the old friend does will ever be so bad that the debt is considered 'paid' by the NPC. Particularly manipulative "old friends" will point out this debt as a manipulation technique.
- "The slippery slope": the old friend is back in town, and wants to relive the good old days of sex, parties, drugs, and booze. It starts with a drink just to catch up, next a night out. No single step seems like it's a huge deal. The NPC is thinking, sure I can handle one drink, or "it's just dinner", and "if I don't look out for my friend at this party, things could get bad, like they did in the old days, but I'm responsible now, and I can take care of my friend".
- there are more, many more
Of course all of this is happening behind the scenes from the perspective of the PCs. All they see is their new friend starting to behave in an erratic fashion, become less reliable (because they're always off dealing some issues the old friend is causing), and eventually distrustful because they are being manipulated by the old friend. Presumably, the PCs will also get distrustful in return. Given you're description of them though, chances are they will make a sincere effort to figure out what's wrong but
- The old friend is in town for *the last big con to get out of the game, and needs the NPC's help, of course it has to be kept a secret
- The old friend brings with them the risk of bringing something deeply embarrassing, even incriminating, to light, so the NPC tries to keep the PCs far away until they can get their old friend to move on
- The old friend is laying low (from the cops, a former criminal associate, a gang boss they screwed over, whatever). Of course the NPC won't tell anyone the old friend is crashing at their place, "just until the heat is off, of course"
- there are more, many more, you get the idea
With this setup, you can keep to your original storyline and not waste all the planning, because the NPC just gets drawn back into his old life. It also sets up a new villain for you, or at least side-villain. You know, the kind who somehow always survives, and manages to weasel out of any kind of comeuppance, because they always have the upper hand in the negotiation; they know something the PCs need to know, or they've planned ahead such that just as the PCs think they have the "old friend" to rights, they have to make a terrible choice (save the innocent girl/boy, or chase after the "old friend"). In other words, the "old friend" becomes the character everyone loves to hate.
Alternatively, if the PCs can really manage to get the hooks out, and save the NPC, that is already a compelling exploration of redemption as a continuous choice rather than a single action (for the NPC) and how redemption requires help (in the form of forgiveness/acceptance) from others (the PCs), and as a bonus, the old friend could become the tragic figure, i.e. end up dead despite the NPC's best efforts. Broken and defeated, the NPC turns to the PC, reveals all; the dark past, the wild parties, the temptation of using again with the old friend, ; maybe they did actually use? Up to you the GM. Either way, you still get to use your storyline and planning.