I'd offer to go out to coffee or lunch with this person, open up and ask if you could be of help. Just opening the door might be helpful. If the person isn't doing well, they are most likely getting feedback from their manager, or feel it in some way. If you open up and show you care, you set yourself up as someone who matters to this person, and then you set yourself up for giving candid feedback. I recommend reading everything by Kim Scott on this. It has helped me https://www.radicalcandor.com/

If this person's poor work is impacting yours, tell your manager. If this person's poor work is impacting your team, address it with their manager. If they come to you for advice on their performance, then give them feedback.