Hi there, I Work in travel and you can still book open ended flights.
As per some previous answers they can work out quite high in price though!!!
My advice to you would be to look out for a "explorer" package trip..basically what this means is that a company , say for example the airtours holiday group, book you a return flight and 1 nights accommodation.You can use the outbound fligth and the first nights accommodation to get you started and then dont use the return flight!!!! When your friend is ready top come home i have been advised that one way return fligths are very cheap from cuba back to the united kingdom. If worst comes to worst and your friend can not get a a return flight from Cuba back to the uk they can always book a flight across to the states and hop on a flight from the states back to the uk which again are very very cheap at the minute due to the exchange rate.
hope this can be of some help to you