With respect to your close friend, "
Full frame is better" only applies for certain use cases. In others, "
APS-C is better" would be more accurate. There are compromises with all formats, as well as relatively narrow areas in which each excels.
There's lots of information on the web extolling and comparing the virtues of different formats for different purposes, so I won't repeat here. What I
will say is, if you don't feel like you're missing out on something, there's no reason to change. All of the Pentax cameras I shoot are APS-C format, with a mix of APS-C and full-frame-capable lenses. I also shoot full-frame Sony for certain use cases. For general photography, I'm far more likely to pick up my APS-C Pentax gear, as the image quality is great and I appreciate the slightly more compact size and lower weight.
There
are situations where full frame
can be better, but it comes with trade-offs. So if APS-C already offers what you need, stick with it