Hi @NicolasK is very definitely wrong.
You, not being from Switzerland, cannot buy the trial half fare card - it is only for Swiss residents or residents of the bordering regions in neighbouring countries (it says so here: https://sbb-tickets.cc/en/campaign/friends/voucher.html).
Furthermore, the half fare card that you can buy (the 1 month half fare card, available here: https://sbb-tickets.cc/en/leisure-holidays/inspiration/international-guests/swiss-halffare-card.html) is apparently invalid. Although it doesn’t say it anywhere on the web page, this 1 month half fare card is actually the half fare card from the ‘Swiss Travel System’, which it says in the ‘conditions’ of the Day Pass for Friends is invalid.
It is extremely confusing and disingenuous on the part of SBB, but in summary, as foreigners who do not reside in Switzerland, you can’t get a valid half fare card for this deal and therefore you cannot use the Day Pass for Friends.
Please be really careful and read all the (very) small print SBB conditions (even if all their big advertising says you just need ‘a half-fare card’) so that you don’t fall into a trap.
From personal experience: I bought the 1 month half fare card from SBB for my 2 guests, along with a Day Pass for Friends (and I also read @NicolasK ’s reply to you to make sure it was okay). BUT a conductor on the train told me those half fare cards (that I bought through my SBB account on the SBB page) were the so-called “Swiss Travel System” cards. She said then the whole thing was invalid and told me I had to buy new tickets for hundreds of francs. It was also really embarrassing since this little revelation that the “SBB 1-month Half Fare card” is actually the “Swiss Travel System” half fare card happened on one of the big panoramic trains. The trip was meant to be a wonderful gift for my guests, but the whole ticket thing ruined the experience.
Anyway, good luck and I hope you haven’t wasted too much money on the Friends Day Passes.