It happened during the 2002-03 season. Henry, who was 25 then, was playing his 4th season for Arsenal. That year he wouldn't win the Premier League (the 'Invincibles' were the season after), but he broke a record that is still intact to this day.
That season he got a whopping 25 assists, as well as scoring 24 goals, one away from Ruud van Nistelrooy. It would have made an unmatchable double.
Since then, no one has managed to get close to this figure, until this season. Kevin De Bruyne's performance invites us to think that he could do it. The Belgian has got 15 assists after 22 matchdays, so it wouldn't be unreasonable for him to make it.
In fact, he could easily surpass the official record, as the Premier League only counts 20 of Henry's 25 assists given that season. What happened to the five that went limbo on the official championship record is a mystery.