lilblue2324 Sims 3 is absolutely not locked to PC or Mac. The (current) problem is that the 64-bit version of Sims 3, which is Mac-only, requires a separate set of "entitlements," as they're called. These are supposed to be applied whenever someone buys Sims 3 content, and were supposed to be granted to all players who already owned the game, but that didn't always happen. However, EA support reps can and should add those missing entitlements when asked.
Point is, please go back to customer support and ask for the 64-bit Mac entitlements. If the support rep doesn't understand the request, linking these master threads may help:
https://answers.ea.com/t5/Technical-Issues-Mac/SOLVED-64-bit-Sims-3-content-not-in-Origin-library/td-p/9653314
https://answers.ea.com/t5/Technical-Issues-Mac/Redeemed-Sims-3-codes-but-wont-download-as-says-only-available/td-p/13789348
As for your original question, the EA App uses the same game library and the same data on the back end as Origin does. And the player's account owns the games, not the computer. So your mother should be able to play on a new Mac or Windows PC just by signing into her usual EA account.
This is NOT true for the console versions of Sims 4 though—that content doesn't transfer to PC/Mac or between Playstation and XBox. (It does transfer between different models of PS or XBox.) So your mom would need to rebuy her Sims 4 content in order to play on her Mac.
If you run into trouble with any of this, please let me know.
