Preferably with dd (https://wiki.ubuntuusers.de/dd/). There may be tools for windows.. but i would not trust them at all.
Thanks. Main problem is: Once the phone is locked - which is the case - it is almost impossible to make a decent dd-image. dmesg shows the device, but the phone was never rooted, so it is basically impossible atm to clone the device. However, I'll give a tool from elmsoft a shot later.
Blockchain.info is a web wallet so as long as you have the necessary details, you can access your coins even if its not on that particular iphone.
Now, Check the details the orginal owner left. Did he left a recovery/backup phrase? If you see a 12 random words enter it here https://blockchain.info/wallet/#/recover.
If its more than that, It must be an old version of blockchain.info wallet, I don't know if this still works but try to enter it here: https://blockchain.info/wallet/forgot-password?guid=
If you don't have that. Check for email and password. You can recover your wallet ID here: https://blockchain.info/wallet/#/reminder. Unfortunately if you don't have the password only the recovery phrase [as stated above] can help you access your coins.