Depends his custom assets and script's (basically anything not by ea) wouldn't fall under copyright law and thus he owns the right default meshes from the game is indeed owned by ea this is a wildly debated legal topic in modding but this is the legality works as far as I know
Taken directly from EA's EULA:
Your Contributions.In exchange for use of the Software, and to the extent that your contributions through use of the Software give rise to any copyright interest, you hereby grant EA an exclusive, perpetual, irrevocable, fully transferable and sub-licensable worldwide right and license to use your contributions in any way and for any purpose in connection with the Software and related goods and services including the rights to reproduce, copy, adapt, modify, perform, display, publish, broadcast, transmit, or otherwise communicate to the public by any means whether now known or unknown and distribute your contributions without any further notice or compensation to you of any kind for the whole duration of protection granted to intellectual property rights by applicable laws and international conventions. You hereby waive any moral rights of paternity, publication, reputation, or attribution with respect to EA’s and other players’ use and enjoyment of such assets in connection with the Software and related goods and services under applicable law. The license grant to EA, and the above waiver of any applicable moral rights, survives any termination of this License.
As I said, he doesn't own anything that he makes as it applies to sims 4. Simply by making the mod for SIms 4, you relinquish your rights to EA as a condition of use.. Not so hard to understand.
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