Hello Hermina
She can buy the Saver Day Pass online or on the SBB mobile app. To obtain it, no subscription is necessary: it is available to everyone. However, it is only available online or on mobile, and the price changes based on the purchase date: the earlier you buy, the more you save, and as it is a special offer, refunds are not allowed. Here you can find more information.
If your client is staying for several days in Switzerland and during her stay will use public transport, I suggest taking a look at the offers for tourists, especially the Swiss Travel Pass, which is a package of day passes for multiple days. This can also be purchased at any SBB counter. Here are our opening hours.
I wish you a nice day,
Chiara_S