Upvote:2
UK visa applications require you to disclose
Some countries (e.g. Schengen countries) make a difference between “cancellation” and “annulment” (for both entry stamps and visas). A cancellation is serious and suggests a refusal of entry, some sort of alert or fraudulently obtained visa. An annulment is a technical measure used if a document was stamped by mistake or a visa needs to be corrected. I don't know if South Africa makes a similar distinction and whether UK immigration officers are necessarily familiar with these nuances but it can generally be inferred from the stamp itself.
If by “cancelled stamp”, she means that she has been refused entry or that the consulate reversed their decision then it should be disclosed and could obviously be a bigger issue than if the stamp has merely been annulled after she reported her change of plans to the consulate herself.