I'm going to assume that by your Truedelta name that you are a woman. It has been my sad experience, as a male, to see women taken advantage of especially in the world of automobile repair. That is not to say that all mechanics are pigs, but many do take advantage of women because they believe, or assume, that women no nothing of automobiles and thus an easy fleece.
Regardless, they took advantage of you. When your battery went dead, your ECU was fine. It does not need to be reflashed or updated or any such thing. Once a new battery is connected, all will be fine. You may need to reset any clocks in your Fiat and/or you may need to adjust some other optional features that may run off the battery and need to be reset. The ECU is not one of these.
If you are a woman, may I suggest that in the future, before any automobile repairs, you consult a male friend for advice. Better yet, being forewarned is forearmed--buy an inexpensive used book on basic modern automobile repair from eBay or Amazon.
No, I'm not suggesting you work on your own car. The vast majority of everyone (including me) cannot do that because of the advanced electronic computer controls that are in modern automobiles. But it would give you an understanding of how things work in your Fiat.
Hope things go better for you in the future.