My spouse and I met while she was here on a student visa, so she had a driver's license and car at that point. We didn't do the K1, we did the immigrant/marriage visa route, so there wasn't the 90 days part. However...
In the almost 3 years we spent waiting for the immigrant visa (she was originally here on a J1, which requires going back to the home country for 2 years before you can adjust your status), we fought A LOT when she got back, even though she had lived here for several years previously! She struggled a lot with not having work experience here, so she had a lot of anxiety about finding a job. She is also very introverted so she doesn't make a lot of friends outside of our relationship, which kind of makes me her only support here, which isn't all that healthy.
I say all this to point out that moving to another country is really hard, even when you wanted to do it and you're not stuck in the house all day. It's tough to understand if you haven't been through it, and a lot of people get incredibly homesick.
Of course Fernanda was way overboard, but she is young and scared and homesick, so I think that has a lot to do with her overreacting. Jon wasn't wrong, but he's got to hang in there through this part and be as available as he can until she gets her footing. It will pass.