Alternative to salt when using Mortar and Pestle
2 solutions:

I have an unpolished granite pestle and mortar: they're much cheaper than the polished granite ones and are much more abrasive then a wooden or ceramic one.

I also don't use any salt, (for taste reasons, not for health reasons), but if your partner is intolerant to the Na in the NaCl (salt) you can get get 100% chemically pure CaCl2¹ from any decent pharmacy² and that is an unlimited allowed food additive according to the FAO/WHO and it tastes just like salt.³


Note 1: Food grade is only 85-90% CaCl2 and will not be sufficient in your case...
Note 2: Ask for the crystalline form...
Note 3: Talk to your physician about this solution before believing random strangers on the Internet!