depends....

If they have buildings with new names at the export_descr_buildings.txt level you'll need to edit to merge the mods. (as you'll want any building capabilities and recruitment of units in that file to be specific to your mod)


If they haven't altered anything at that level you might have more luck. Check though that neither your mod or theirs introduced a new culture - some bits are culture specific, and references to a non-existant culture or missing a plan for one will mess things up. Also did they change buildings for cultures you want to stay the same? That might take some unravelling.

You'll need their descr_items.txt, items.db file and items folder as well btw, unless you're re-making those.