I'll answer it for him.
The UK's economy will be knocked sideways. Since Friday when the Brexit camp pulled ahead in the polls, we've seen £76bn wiped off the value of FTSE 100. That's just the FTSE 100 - not the whole market - and that's just on a *possibility* that Brexit might win. The markets will crash if Brexit win, and if you think that's pure speculation, it isn't - there's very good reasons why. First, it means that we'll have years of uncertainty ahead with businesses not knowing what the trading relationship with Europe is going to be like and therefore unable to make strategic investment decisions. investments will be put on hold and growth will stall. We'll probably fall back into recession, at least in the short term. In the medium term, businesses that have invested in the UK so that they can export to Europe will review there strategies and some of them will decide to shut plants or move production overseas. If the reason they came to the UK was in part because the UK is in the EU and is a gateway to European exports then if that ceases to be the case, we can expect all of these businesses to stay put, can we.
Professor Minford - just about the only notable economist the Brexit camp has managed to field - said in 2012, “Over time, if we left the EU, it seems likely that we would mostly eliminate manufacturing <from the UK>".
International confidence in the UK's economy will be dented and the value of the pound will fall considerably. Whilst this will actually help our exports, it will push up the costs of our raw materials, food etc. Inflation will rise, and we'll probably have to put up interest rates. Costs of mortgages will go up so people will have less money to spend and that will depress the economy further. Labour costs will increase, also fueled by a decline in EU migrants filling the low paid jobs, leading to increased labour shortage (we ALREADY have more vacancies in the economy than unemployment). HIgher costs makes our productivity even lower (it's already much lower than the world's leading economies), making our products less competitive and sales will decline. We'll struggle to meet current budget commitments (actually we'll simply fail) and since we don't want to increase borrowing (it's already perilously high and failing to keep it under control would result in our credit rating being downgraded and interest rates going higher still), we'd have to make even deeper cuts to public services and extend austerity for several years longer than planned.
If we are ever to recover from this, it will be through increased international (non-EU) trade, but it will take years to negotiate all of the trade deals we need and even then there's no guarantee that these currently non-existant deals will yield enough extra revenue to plug the gap.
On the EU side, they'll lose a chunk of exports to the UK since their goods will be much more expensive, both due to the exchange rate and the tariffs that will inevitably be applied. The UK's leaving will very likely fuel the desire for referendums in other countries, particularly the Netherlands and France. Conceivably, countries such as Italy which are already in a precipitous position could be pushed into bankruptcy. If the Italian banks were to fail, the amount of money needed to support them would be many times greater than that for Greece and with strained finances in Germany already (heightened by their loss of exports to the UK), they would have no choice but to allow Italy to leave the Euro and devalue. Greece would surely follow, and then Portugal and Ireland. The Euro would be dead. If that happens, the whole of the EU is thrown into turmoil and that alone would take decades to recover from.
I could go on.