He DTRHFF. Thanks for posting in the community, to answer your question though, yes you will be able to open their company file. But, since you have a 1 user license, you will not be able to open in multi-user mode.
Also, it's important for me to mention that since you have 2020 and your client has 2019 versions of the software. Once you upgrade the company file to 2020, it will no longer be readable in the 2019 version.
Let me know if you have more questions or concerns, I'll be glad to assist you further. Have a good day.
