If you're in PC, you can modify the savefile which is stored in appdata/local/PVZ_ND/savefile.ini
If you need help you can join my discord server
Viewing post in Plants vs Zombies Neighborhood Defense comments
I appreciate the invitation to the discord server, however I'm super reserved on the servers I join, mostly just ones made by my friends, I also must say that I'm very thankful for telling me about such, I'll have to just tinker around to see how will I be able to unlock things to then pass the edited copy of the save file to my friend so he finally stops complaining about the difficulty, Which btw, I love what you did with it, it's extremely difficult but not enough to be unfair or overwhelming (it's a bit unbalanced at times but that doesn't matter)