I wanted to install one game on my friend's Nokia 3650, but it's imposible because it says that it has low memory. In Fexplorer says next: Z: (0/18240 KB). What could I do to free some ROM memory? Restart Won't help.
You can't free ROM it is, as the name implies, Read-Only Memory (not-writable, not-deletable, just readable).
What you should free is RAM (by closing any apps you may have running in the background) and then free up space on the C-drive (apps installed in phone memory, or by moving pictures from phone memory to the memory card, etc.) or the E-drive (the memory card, MMC) depending on where you want to install the game.