2Ti 1:15-18, 15 This you know, that all those in Asia have turned away from me, …16 The Lord grant mercy to the household of Onesiphorus, for he often refreshed me, and was not ashamed of my chain; 17 but when he arrived in Rome, he sought me out very zealously and found me. 18 …and you know very well how many ways he ministered to me at Ephesus.
Paul’s many friends in ministry disowned him in his hour of need primarily those in Asia as he notes here and later in 2Ti 4:16 ”… “At my first defense no one came to stand by me, but all deserted me. …!”. But one man, Onesiphorus, a true and faithful friend often visited Paul in prison and refreshed him with his presence, encouragements and support. He was not ashamed to be associated with a prisoner, often putting his life in harms way to visit and encourage Paul. Think of the sacrifices, he often traveled from far to find Paul never giving up. He also at Ephesus had ministered to him perhaps financially or housed him whenever Paul was in town. This man had been very kind to him and Paul acknowledges him.
For his knoweleage, Paul sought to make Timothy aware that people will come into his life and most may leave. But as in chapter 4, God will will not leave him in his hour of need. There will be from time to time faithful friends like Onesiphorus who will be beneficial to him that he acknowledges and return kindness. A good man will seek opportunities of doing good, and will not shun any that offer assistance.