The problem for the 18 year old is that the age for adult travel in Switzerland is 16 and 14 in Italy.
So she gets an adult ticket I'm afraid
From the SBB information page:
Children and young people pay nothing for a journey on the EuroCity
between Switzerland and Italy on Swiss sections and 50% of the adult
fare on the Italian section. The offer is valid for children and young
people up to the age of 15.99 with a Junior Card or Children's
Travelcard when accompanied by an adult aged 16 or over. Up to four
children and young people per adult companion can travel at the
reduced price.
The price is valid all year round per person for a single journey on
EuroCity trains from Switzerland to Italy and vice versa.
Bimbi Gratis EuroCity. Conditions:
Limited offer. For groups between 2 and 5 people with at least one
adult and up to 4 children under the age of 15. Advance booking is
possible starting 180 days before departure. Reservation compulsory.
The ticket is only valid for the booked day and train. Exchange for
another day or time possible free of charge, subject to availability.
Refund possible before departure for a fee. No refund after departure.
Discounts:
Adults receive a 40% discount on the normal fare. No further
reductions with Half-Fare Card or GA. Accompanied children under 15
years of age travel free of charge.
Within Italy. Frecce, Intercity and Intercity Notte. 0–under 4 years: children travel free of charge (without their own seat).
4–under 15 years: children receive a reduction of 50%. Regional
trains. 0–under 4 years: children travel free of charge. Trenord:
4–under 14 years: children receive a reduction of 50%. Regional trains
Italy: 4–under 12 years: children receive a reduction of 50%.