So, he can load Harmattan just fine, but not Nitdroid?
Is this first time he tries to boot Nitdroid, I mean has he ever before done this successifully and just now started to fail; or never had it work before?

The log on failing to mount /dev/mmcblk0p3 is suspicious, but I believe it has nothing to do with this problem if he can boot Harmattan OK; The kernels are anyway on /dev/mmcblk0p2 and the ubiboot config files on /dev/mmcblk0p1 so booting any OS should work just fine.
(And since /dev/mmcblk0p3 is the Harmattan /home mount, if Harmattan works then it sure is mounted correctly at OS boot...)

Now if I am correct in thinking that he never managed to run Nitdroid before, then the problem is clear;
There is some problem in the Nitdroid setup, for example:

• the /sbin/preinit_nitdroid file is missing, or has not r&x attributes set
• the Nitdroid filesystem image has not been extracted correctly to /home/nitdroid/
• the nitdroid image is wrong or was corrupted at download