Originally Posted by JacobNL

As far as I know there's no bus gates being used in BRU as of now.
Since you are arriving from a Schengen country, you'll enter the Schengen airside area of the airport and you won't have to clear security. There's no guarantee you'll enter the airside area be at the very exact gate where your friend will be queueing to board his flight. But if you do your best to disembark first and your friend to board last, there's a fair chance that you'll be able to meet each other if you do proceed towards his boarding gate as soon as possible.
However personally I'd definitely have a plan B ready if I were you.