Thanks for contacting me. I'm happy to hear that you enjoy my app.
I've been often asked if there's a version of Custom Maps or something similar for iOS, but as far as I know there is not. I have never owned an iOS device so I have not done a thorough search, but nobody has come back to tell me that they have found such an app.
But Custom Maps works also on very old Android devices, so maybe you can find a discarded/unused old Android phone that your friend could use and install Custom Maps on it. Custom Maps also doesn't require data connection other than at the time when creating a map which can be done at home using wifi connection, so the phone wouldn't even need to have a SIM card in it.
Sorry I can't be more helpful.