making the material master description non-editable in MM02 is customizing, and this customizing is controlled by six different field selection references, So it certainly possible to create a setup that allows your external party to use MM02 for a change while your internal user cannot do it.
Read OSS Note 199165 - Field selection control apparently not working
This should give you an idea how you can control it.
Alternative, you can try if you can change the description by using the stone-old transaction MMZ2