I don't think it'll be that in depth of an interview.
Any experience in customer facing roles would be good, even if voluntary. So customer service. The ability to multitask and recall information given. Knowing the importance of hygiene, appearance, cleaning up after oneself, not being afraid to muck in, deal with say fish orders, washing up, making sure she maintains cleanliness where possible. Flexibility for shifts would look good, willing to work extra if required.
An eagerness to learn new skills if this is a first time in such an environment. Ability to work as part of a team, but also by herself, along with having no trouble taking orders from senior staff. Being proactive so making sure areas are clean, anything they're low on, say salt is running low, or she notices the mushy peas are low, either rectifying herself where possible or advising someone more senior.
Basic interviews ask for what you've done so far, what interests you in the job, awareness of basic hygiene & food prep, possibly 3 good traits she has, 3 bad traits. Then maybe a scenario involving a feisty customer, what would she do. Even though computerised, they may ask some basic maths questions. So if the bill came to £3.50, they paid with £5, how much change.
I would add in vigilance when it comes to money and notes in case of counterfeits.
Hope she gets on well.