The demographic outflows as you phrase it started with the quiet revolution in the 1960s and 1970s. I would suggest that you look at the 1980s and 1990s in this wider context and as a continuation of that. The FLQ crises is what people focus on but there was a lot more going on. I have much more information but no sources on hand so I'm going to leave it at that.

Your coworker certainly is correct that Montreal used to be the centre of Canadian fiance. However it was not just English Canadians moving on mass, more to the point several companies moved their headquarters or main canadian offices.

There are two parts to it: first, anglophones themselves and second enterprises.

Anglophones did move en masse out of Québec and although the numbers of them leaving the province were higher around referendums, it had been a long them trend since at least the 70s.

One of the reason is that the English used to have a strong grasp on the Québec economy. They controlled most of the big and medium businesses in Québec. But following the Second World War, there started to be a consciousness of their place in society, the government started to get involved in the economy and the Québécois started to expect services to be in French and have it be protected by the government.

The diminishing grasp of the English on the economy as well with a a State requiring things to be done in French allowed a new class of French elite to rise thanks to initiatives like Hydro-Québec and others. Due to all of this the English started to look elsewhere and many left. Overtime, the importance of French also made a lot of anglophone feel like the political and social climate was hostile to them, most of which who refused to speak French in a French province. Younger anglophones in Québec today are very likely to speak French but it was not the case back then and a lot of them left as the climate was not as good for individuals trying to live in an English-only environment. That being said, a lot of it was perception rather than fact but that was enough for them to take the decision to move in the United States or in the rest of Canada.

Regarding entreprises, the most famous case is the SunLife Financial announcing that it would move its headquarters in 1978 (around the first referendum). Following the decision of the provincial government to institute Bill 101 to protect the French language, SunLife decided that it would move its headquarters to Toronto. It was not the first company to do so and many more would after that as well. But it was the most symbolic. Some companies started to send their operations in third-world countries (Québec had a lot of factories producing clothes; those were shipped to Asia) but high-end industries like finance did move to Toronto to some extent. Overtime, the proximity to the United States and political uncertainty in Québec led industries to move to Toronto.

Now, how bad was it? It is definitely blown out of proportion for political gain. Companies did leave but many did not. Some of those who did were looking to restructure anyway (like in the case of clothing factories) and others were simply already looking into leaving for Toronto for a bunch of unrelated reasons. The referendum only gave them the little "push" in an otherwise existing trend. Interestingly enough, Toronto has a lost quite a few headquarters as well since some moved westward. So Montréal was nothing special; a small uptick at best.

So is your coworker wrong? Not really but he blows it out of proportion. First, Montréal started losing "feathers" in the 60s regarding finance in Montréal. Most of the industry left around the first referendum. Some more did around the one in 95 (like CP although not in finance) but at this point, Toronto was already established as the canadian centre for finance and most losses in Montréal were just part of a trend with maybe a small uptick.

Regarding anglophones, many followed but it's larger changes in Québec society at-large that led them to want to leave rather than the referendum themselves.

Some sources below. Note that anything Québec referendum is extremely difficult to study as the subject is still vividly in the collective consciousness. Moreover, in academic circles, the matter is particularly heated still to this day with scholarship that tends to be a tad too obviously one-sided.

Statistiques Canada regarding language trends. See (amongst others): http://www12.statcan.gc.ca/census-recensement/2011/as-sa/98-314-x/2011001/tbl-fra.cfm. THat being said, using the same data, you can find a more palatable version on Wikipedia.

Institut national de la Statistique for corporate headquarters info

Papers of the CRIC and Référendum volé for broader matters and questions.

Newspaper clippings for stuff like why companies left, trends etc.