E-gates are valid only for holder of some country passport: UK, EEA, AUS, CND, JP, Singapore, South Korea, Switzerland, USA (https://www.heathrow.com/arrivals/immigration-and-passports). I didn't check but usually only on some passport, but usually it is the case for most people (and our site) [diplomatic passport often need regular check]. You may need to manual check also if you are required to get some stamps (this depend on the status you want to enter).
Note: it requires passport (it is not about citizenship): I often travel without passport, so I need the manual check.
In any case: if she is eligible, she can try. Worst case she is flagged to make the manual check: there is not just a biometric check, but also passport (and eligibility) check. She may be in a special database which flag her to manual check, just to be sure her intension (a machine cannot yet do it).
As other wrote in comment: not always the e-gates works, and as far I know, some people are sampled to do a second check.