If this person doesn't report to you, you're best off not doing anything without their manager on-board. Otheriwse, leave it alone. I'd frame it in one of two

If this person doesn't report to you, you're best off not doing anything without their manager on-board. Otheriwse, leave it alone. I'd frame it in one of two ways... If the manager doesn't know this person is under performing, why? This is a process failure that needs to be rectified, work with everyone who needs to be included on coming up with process changes that will prevent this in the future. Code reviews, retrospectives, 1:1, team reviews, whatever methods seem like a good start. Addressing the underperformer, what would you suggest for bringing them up to par? What is it, actually that's not up to par? Are they slow? Sloppy? What can help with it? Working more closely with them? Pair programming more frequently? Daily/Weekly/Per Ticket code reviews? Pull requests? Break it down in to actionable bullet points. If they're too slow because they over engineer, suggesting more time pair programming would be a great idea. If they're struggling with a given technology, maybe an investment in a course could get them to speed? Maybe they get a little analysis paralysis with tasks that are to open ended and undefined. Suggest pairing them with a more senior resource to tighten the mental boundaries and keep them moving along. I always make it a point when coming to others with a problem to have some suggestions towards rectifying. Now you're not complaining, you're trying to help. Don't expect your plan to be THE plan, it's meant as the conversation starter, and now everyone involved has a stake in a successful outcome. If their manager isn't responsive, or you can't put a finger on what's off about this person's work, you don't have much you can do.