A trim 12 valve cannot by nature meet that spec, and therefore they should have ordered something else.
My NACE valve are always stressed relieved, use NACE compliant bolting and are checked for  hardness check. In all these years we have never had this complaint. So I am thinking that the customer should have ordered a different valve in the 1st place.
Anyway, as Trim 12 is essentially 316 and, therefore, comes under Table A.2 of Technical Circular 1, 2007, you will note that you have two options:
1.  max 60 deg C and/or max 1 bar a partial pressure H2S
2.  maximum of 50 ppm chloride in the fluid
In each case, the material has to have a maximum hardness of 22 HRC.
So, how does your material fail to meet the requirements?
Either way, it is up to the end user - not you - to work out whether the valve materials and the environment they will see meet these restrictions.
NACE MR075 compliance is completely dependent on environment
If you have conditions that allow the use of carbon steel and  cast steel valve. Then you should be okay neither material should be over 22HRC.
One caution that probably is'nt an Issue with an LCC valve But may be a problem with WCB or WCC valves. Some manufactures use solid 410 stainless as a standard for wedges and backseat bushings on Trim 8 valves. if this valve was converted from trim 8 to trim 12 the 410 parts may still be in the valve with 316 or stellite overlays on them. If that is the case the 410 stainless is not NACE compliant.

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