Adrienne Thank you for the update. Your theme developer added the Google MAP API requirement in his code or they always had it and your user was changing the way it was used? Everyone wants to make their product the best. Unfortunately we cannot strip down SLP so that it is no longer a MAP and LOCATOR plugin that would mean it is not what it is advertised to be. SLP has ALWAYS used javascript and Google API. Google never required an individual site API key before , and any domains already in existence prior to June 22 2016 were grandfathered and not required to get a Google Map API key. Regradless if you had two plugins or a theme and a plugin both using the Google API there would be a conflict , now Google is making it obvious , i.e by showing you the ERROR MESSAGES. The SLP author and developer knew that Google was heading in the direction of requiring MAP API keys and added in the line where you added your Google MAP API key. We keep up to date with any changes to Google geocoding, MAPs, APis jquery , WP codex and requirements because its the business we are in. He has been doing this for many years and is not a fly by night, Many plugins become orphans.
https://docs.storelocatorplus.com/blog/q-my-map-is-not-showing-up-what-did-i-do-wrong/
Your theme developer is not in the geocoding and locator business primarily are they? Either way, pointing fingers is futile. We did NOT change the Google MAP API key or how the jquery worked. Since you are keeping that theme there is not much of an alternative regarding SLP or any other locator /map plugin that uses google MAP API. We always suggest that any site, (especially one with clients), have back-ups and/or a virtual site to test on before updating themes or plugins. That way you can see what changes will happen and roll back if needed.
Now we are off subject of this post and I will need to close it.
P.S. If you have multiple clients using SLP, you are required to have the Premier Subscription.