Hello Puttz95
Thank you for your request.
The Swiss Half Fare Card is valid on Swiss routes. There you have a 50% discount. When travelling with TGV trains (to Paris), you pay the full price from the border. It is best to split your bookings. Zermatt - Geneva with Half Fare Card, Geneva - Paris without discount. From Italy to Switzerland, the discount applies for the trip, when travelling with EuroCity trains. Meaning you could book Milan - Zermatt direct with Half Fare.
I wish you a nice day.
Kind regards, LeaG