The oncologist is just listing possible scenarios. That does not mean that all of this will happen. You also have to distinguish short and long term effects. Both of these are different depending on the person and also on the what exactly is irradiated. Especially the latter is quite variable.
In the short term the radiation is miserable (after 3-4 weeks) and gets more miserable after it is finished. Sores, pain, no taste, mucous from hell etc. However most of that will pass. Taste generally comes back but it can take a long time. She might end up having a dry mouth, this is less of an issue with imrt but depends greatly on what exactly was zapped. There are several saliva glands (3 major pairs) that are in different locations.
That said the radiation is the life saver unless they were able to remove all the cancer surgically. Look at it this way yes it will make you likely miserable on the short term and there MAY be long term effects. Dry mouth, dental issues, altered taste. But look at it this way even if you have long term effects, you have to be alive to experience them! Radiation will give you that chance.
She will need effective pain control, mouth rinses, viscous lidocaine etc
At this point you have plenty of time to get ready. Have her eat plenty of her favorite foods now. After 2 weeks of rads it will all taste like wet wallpaper.
Check out the ocf website there is plenty of info there.
M

Boy you guys are quick. ......agree with all of the above