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No, that would be to good to be true
See the are of validity map and select the 3rd option (“Saver Day Pass / Special Day Pass … without half fare card”). Then zoom into the Jungfrau area: You’ll see that the Jungfrau railways are in gray, which means no reduction, see also the legend behind the “i” symbol in the top left box. The Friends Day Pass includes free travel until Grindelwald or Wengen.
With the Swiss Travel Pass (NOT “Swiss Pass”), the discount on the Jungfrau railways is actually only 25%, not 50% as with most other scenic / tourist mountain railways. In that top left box on the map, switch to “Swiss Travel Pass” to verify. For 50% on Jungfraubahn tickets, you’d need some half fare card or pass.