There's no 100% security against physical access. The data on the device is not encrypted and it can probably be read without knowing the code using the flasher utility, USB and some inside knowlege about how N810. However this or installing other nasty stuff such as software keyloggers is not trivial and cannot be googled. If he tries this it will take time and programming knowledge.

I think the tablet can serve as a secure communication device if passwords arent stored locally and all communication is encrypted.

The big advantage of chosing N810 over a regular computer is, that probably neither for its hardware nor for its software any malware is available. That is, if you want to install a keylogger (either hardware or software) on the N810 you have to do it manually.

Cheap alternative: Boot from a Linux Live CD like Knoppix. If the hardware cannot be manipulated, I'd consider this equally secure. (Of course, any computer can be compromised, but not by anyone. I think, it's difficult to install a hardware keylogger on modern notebook.)