Hi Jules,
It's hard to know what is causing your M-in-L's nausea, but it could be the Tamoxifen, in which case it may help to take it with food e.g. at a meal time. Otherwise a lot of people find ginger (biscuits or gingerbeer) and/or carbonated water help with nausea. If she mentions this to the hopsital, they may well give her some anti-sickness medication. Hopefully things will soon settle down for her and she will feel better.
I'm sure she is very grateful for your support.