What information did Colonel Fitzwilliam give about Darcy regarding Bingley that upset Elizabeth?

Fitzwilliam tells Elizabeth that Darcy orchestrated the separation of Jane and Bingley because she wasn't suitable. 

What he told me was merely this: that he congratulated himself on having lately saved a friend from the inconveniences of a most imprudent marriage, but without mentioning names or any other particulars, and I only suspected it to be Bingley from believing him the kind of young man to get into a scrape of that sort, and from knowing them to have been together the whole of last summer.’
‘Did Mr. Darcy give you reasons for this interference?’
‘I understood that there were some very strong objections against the lady.’
‘And what arts did he use to separate them?’
‘He did not talk to me of his own arts,’ said Fitzwilliam, smiling. ‘He only told me what I have now told you.’