The last time Bing was aware that Darcy liked someone his attempt to help didn't help very much at all, in fact it ruined it before anything even started.
But Darcy's never been one to hold grudges, and after Darcy was done licking his wounds and showed up with a really good vintage scotch at Bing's door he knew he was forgiven. This didn't stop him from promising to not interfere in his friend's love life in the future and before they both got too drunk, he said he'd sign an NDA (not legally binding of course, it'd be more symbolic than anything) that he'd stay as far away from the situation as possible, it's actually the last thing they both remember about that night.
So one week into the Bennet sisters stay at Netherfeild it wasn't really a surprise to him when Darcy brings him into the lounge in the middle of the night and presents him with one not so legally binding document about one Lizzie Bennet. Bing's known for a while and he's been waiting for this moment, in fact and he's planned exactly what he's going to say as he signs his mouth shut on the matter.
"You know if this all works out, we could be brothers."
Darcy simply smiles in reply, and taking the papers and turning his back on Bing, he pauses for a second. Moments later as Darcy leaves the room, he says with emotion in his voice and wetness forming at the corners of his eyes, "Bing, we already are."