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A person with a UK entry clearance arrives at a control point without a return ticket, will it be OK?
Return tickets are not mentioned anywhere in the rules. What is mentioned is the IO's entitlement to be sure that the person has the capacity for onward travel to a place outside of the Common Travel Area; and capacity is best established by showing that a ticket has been bought and paid for, but not always. That line of enquiry is mostly part of the landing interview for non-visa nationals like Canadians and Americans anyway.
The fact that they have an entry clearance tells the IO that the government has already established that the person has sufficient capacity (or they would not have qualified for the visa in the first instance, right?). So it's not going to be something the IO wants to waste time with because it's simply rehashing what somebody else already approved. Instead they will move on to other stuff, like if the person is still employed and if they are going to be safely accommodated during their visit.
If the topic of a return ticket arises incidentally through some other line of enquiry, the outcome will be solely reliant on two things: personal impact and articulation skills. So if your friend is having stress about it, she can undergo a mock landing interview with one of her colleagues. Having 'insider' access to travel arrangements is a truly great premise all other things being equal.
Also, the 'canonical' advice for people arriving on an entry clearance is to bring all the stuff they used to qualify during the application stage.
Adding: there's a related article at UK Immigration officers arrivals interview questions that takes up what non-visa nationals can expect