Customer bought a model from me but says he cant open them, error code is for wrong program used to open.

I think you should write to your customer and tell them that they can try opening your file with another software, and that in case the problem still exist, they should contact you again and you will find another way to solve the issue.

Here's the instructions you can send them:
https://www.instructables.com/answers/How-do-I-open-a-Certificate-Trust-List-file/
It's written there: 'Whatever software came with your 3D printer. If you downloaded an STL file, then you could also open it with Autodesk 123D, Sketchup, or some other 3D modelling software to see what the object would look like. '

Contact CGTrader Team (info@cgtrader.com) if the issue won't resolve.

That's exactly what I did last night when he first told me the error and i haven't heard back. Hopefully he has got it working now, it looks like it's his first purchase so I think he just didn't have the software to view the files so I told him the free ones to try and see if they work too.