Questions about license

There is a company that sells modified libreoffice in its products.

I asked them for the modified source code. However, they answered that they only provided source code to customers who purchased their products.

I think this is a license violation, what should I do?

What makes you believe that they violate the license? MPL requires that those tho receive the binary form must also have means to get source code (see MPL v.2 clause 3.2).

They have specified the libreoffice’s license as LGPLv3 on their third party attributions document. And they have specified that should contact them by e-mail about the modified code distribution.

But when I contacted by email, I could not get the source code.

Is it ok to use libreoffice like this?

LibreOffice is dual-licensed (MPL+LGPLv3), so their choice is OK.

If they distribute derivative work in binary form, then those who get that binary form must be given means to get the source code of the software (including changed code, but the license doesn’t put outer software that is compiled with LO libraries also under that requirement). So, if you obtained the binary form, they MUST also provide source code.