I just replaced my sepentine belt on my Hyundai accent, and I noticed there was a small amount of back and fourth play in the idler pulley in the direction of the pulley , orthagonal to rotation axis.
My friend says that is actually normal for some of the new types of plastic idler pulley's on cars nowadays.
Is this correct, or should it be replaced?
It's hard to say, because "a small amount" is a very subjective measurement. Is there going to be movement as you describe? Most likely yes. The question then becomes, how much is too much. Without being there in person and seeing it myself, I'd suggest your friend is probably right for the most part, though I don't think it is "just" plastic pulleys. Any idler or tensioner pulley is going to have a "little" play in it.
There are two checks you can do to see if it might need to be replaced. First, with the belt in place, look at the belt alignment across this idler pulley from the side. Does it look to be deflected where it crosses the pulley? It is usually very easy to see if the belt makes a slight jog when looking at it from the side. Your next test should be the listen and feel test. If you can turn the pulley by hand and it feels smooth as well as you don't hear any type of noise, the pulley is probably just fine. If either of these tests are not good, then the pulley should be replaced.
