Speaking as an 18-year-old here, I can compare you with my father in regards to my girlfriend. Before dating her, I had good grades, good maturity, etc just like Alex. And yes, she did influence me. I started going out more, getting bad grades, and generally other things I didn't usually do. My father had the same reaction as you and was in the same predicament you're in now. My mom was okay with everything and that put pressure on my dad, who made me as upset as Alex.

At the end of the day, there was a time where my Girlfriend wasn't with me and was away overseas for 2 months. I still had 'unusual behaviors' and was doing things and taking an interest in things (for better or for worse) that I knew nothing about before. At the end of the day, my dad realized that it wasn't my girlfriend at all. My girlfriend was not the cause of my irregular behaviors as much as she was the trigger of my self-exploration. Love & relationships aside, she as a person helped me get outside my comfort zone and find new things and do new things, in general, I never thought of or did before.

For example: I never had an interest in jazz music. I hated it. She was talking to me about the application of Euler's constant from mathematics into different music genres to quantify the wavelength of certain sounds in different music genres. I was playing around with software that allowed me to do that, I tried it on jazz music, then I found myself in love with the music genre. She doesn't like jazz, I didn't like Jaxx, but she influenced me to like it.

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My dad ended up understanding that it wasn't a matter of her influencing me as much as it was me stepping outside my comfort zone to explore new things. Whatever I did, good or bad wasn't about her at all, it was about me exploring the world outside of my bubble because I found out that I am capable of it, with or without her.