Concord rough idle

A friend has a 2000 concord 2.75 that just turned over 100,000 miles. She brought it by yesterday, the check engine light came on and it's idleing rough. Had a code p0305 (missfire on #5 cyl.). I put a set of plugs in and a new coil on #5 cyl then cleared code. She called today and said it's doing the same thing it did before the work was done. Any one have an Idea of where to look at this point?