OK, your Ubuntu boot should look like:
kernel /vmlinuz root=/dev/hdb1 ro
The above is assuming that you have both HD on the same IDE cable. If not, change root=/dev/hdb1 to root=/dev/hdc1.
You can also boot Fedora and mount the Ubuntu partition to look at the grub configuration that it had created, and copy the relevent portion to the Fedora grub.conf. For example:
mount -t ext2 -o ro /dev/hdb1 Ubuntu