Questions about EA vs. Steam and Expansions

So someone played The Sims 4 on Steam and had all their expansions on there on Windows. They bought a mac and now do not have access to their expansions because they purchased them on Steam. I was wondering if I bought the expansions they had for them on the EA app, would they launch with the base game if they opened it on Steam, or would they have to get the base game on EA too? If so, would they lose anything? I don't really know anything about The Sims because this would be as a gift so any help would be much appreciated, I can't seem to find a clear answer anywhere