If someone wants to start an online currency exchange trading cryptocurrency only.
The site is not trading cryptocurrency for USD or any other currency but cryptocurrency.
Do they need to register with FinCen in the US?
I believe only if you touch the dollar. crypto to crypto may not need registration, this was an old believe but it may not be accurate since the laws have changed in unbelievable ways just in the last year. Also Fincen only applies in the USA and in the so called friendly countries, in truly independent nations Fincen does not have jurisdiction. Fincen overseas will only be enforce for very large sums in non interdependent nations, for example Liberty reserve was based on Costa Rica but since Costa Rica is not an independent nation Fincen rules could be enforced.