Former flight attendant here.  My son was a great baby and I began taking him cross-country when he was 4 months old - but I would NEVER EVER consider the roughly 20-hour travel day from CO to Italy with a child that young.  You not only have issues with pressurization, which is severely uncomfortable for the little ones, but then there's the whole check-in procedure, wrangling with luggage, sitting in the car seat/lap for hours upon hours, as well as disruption to the schedule and sleep cycle from being on the other side of the world.  This is all torturous for a young baby and SOOOOO unnecessary!!!! I'd be pissed if my SO even suggested it.

Maybe she'll have a change of heart when you present to her that this is not a good idea at all because of the BABY.  Sorry, but a good parent wouldn't put their infant through that, especially for someone they don't have regular contact with.