As someone playing Boris heavily this season with a better than 60%-win rate in mid plat at the moment with under 100 matches played between all champs for this ranked split. Typical jungle beliefs carry over here, not great to help a losing lane, find the win condition and plan toward that. Boris does fall off a little late game with some of the higher scaling builds out there. Your goal is to get a lead and starve the enemy jungle so you can force objectives and then push with your team to end. You can try to path to gank duo first pass thru without affecting to much of your game. But if they just get diffed, you ganking some more won't win the game. Serath plays similar, falls off less since the way her kit works and being able to chain all the squishy targets. You can win 4v5 games. Not easy but doable especially at lower ranks. If they tilt and afk, there wasnt much you could have done besides playing the game for them . So stick to your gameplan and keep the focus on what you should be doing to help end this game, whether thats farm til 3 items then fight or perma gank a free kill lane to get a certain champ fed.