Horse hockey "it won't." It will have ramifications on the global economy, and those ramifications will be negative, and those will help Trump.
The actual "Brexit" won't even happen until well after the election. I'm reading it could take as long as two years for the UK to fully withdraw.
If anything the immediate impact will be POSITIVE for the US economy since the dollar is now worth a lot more in comparison to the pound.
A strong dollar isn't good for American exporters. It makes American-made goods more expensive overseas. Imports to the U.S. would get cheaper, but we don't exactly need deflation right now. I guess it's good if you're in the market for a Jaguar, but most Americans aren't. Trade with Great Britain is pretty balanced - there's not much of a deficit or surplus.
Anyway, the currency exchange rates always fluctuate. The Pound is probably oversold right now. The situation will correct itself over the next week.