The "metal" taste is normal, to be followed by wallpaper paste taste etc.
Yes, taste is important, but it is better to be around to complain about it. Plus it will improve again.
It is normal that things get miserable. Make sure that she eats enough, if required through the PEG tube.. after all this is why you got it...... no metal taste that way!
As was pointed out it will get worse before it gets better, but it can be done. What helped me was viscous lidocaine which numbs the mouth for a while so you can eat/drink "normally" ... sorry stuff still tastes like crap that way, what stayed around was vanilla.

Unless the RO says that she cannot complete her radiation schedule do NOT let her quit. There is a reason why it takes that long. You absolutely do not want to do this a second time.

Best

M