imho, not your girlfriend's fault
If overdosing killed the fish, it wasn't your girlfriend's fault. There is absolutely no need to dosing your tank because you're gone for a few days. You could probably go weeks without dosing and be fine. In fact, given the overstocked level of your tank, and the undemanding plants in there, the fish food is almost certainly sufficient "dosing." If you were concerned about nutrients, a small water change would have given the plants a bit of nutrients.
Next time you're gone for a few days, just let the plants and few be. They'll be fine.
Also, I strongly agree with the suggestion to get an oto and a shimp, instead of betta and a cory. You are going to need to put too much food in the tank for that size of tank, and it is going to promote algae growth. It may be fine for a couple weeks, but in a month, look out.