the youth friend pass is valid as far as Domodossola. So you need a ticket to Milan.
Now it depends a little on how exactly you want to travel to Milan. There are 2 routes, one with the EC and one with the regional train.(RE)
Reservations are compulsory on the EC and you must have a reserved seat from Domodossola at the latest. You can buy this via sbb.ch. Of course you can board the EC in Spiez and travel just your seats while only be reserved starting from Domodossola . The cost here is around 25 francs per person.
With the Regio you have to change trains in Domodossola and need to have a valid ticket from there. You can also buy this via sbb.ch. The cost here is around 11 francs
The purchase is only possible via sbb.ch and not on the app