Don't be put off trying Aricept. The side effects you are worried about are common to all three of the types of this drug - they all work in approximately the same way, although the exact mechanism is slightly different. The leaflets will all say the same thing.
Indeed, things like nausea or stomach upsets are extremely common side effects and many drugs will list them as potential problems, but most people will not experience them, a few will have them but temporarily and mild, a very few will have severe and persistent side-effects which warrants assessment by a doctor.
Just because your mum has experienced problems with Reminyl does not mean that she will get them with Aricept, however.
It is common practise to switch to one of the other two available drugs if Reminyl proves to be unsuitable.
The usual pathway is to try Aricept first, because it is the cheapest to make and is the least likely to cause side effects. If that doesn't work out, then Reminyl or Exelon can be tried.
It is also possible to have Exelon in patch form. This delivers the drug in a slow, steady day through the day, and for some people this can also help to reduce side-effects.
So please don't give up yet! There are lots of things that can be tried.
Sickness that has persisted for more than a couple of weeks is really too long. I would go back to whoever prescribed the Reminyl, or to your mum's GP.
Usually, this sort of side effect is fairly rare and mild, and abates after the forst week or two. This is called building a tolerance. Sometimes it is necessary to prescribe anti-nausea medication whilst this happens, but if the effects are severe or persistent then it is a sign the drug may be unsuitable, in which case, it will be stopped and possibly a different one tried.
Most patients are started with a half dose for a month to six weeks, this reduces the possibility of side effects and their severity if they do occur, and allows them to build up a tolerance before trying the full dose. This process is sometimes referred to as "titration"
As I said, if your mum is still getting sickness three weeks in to the half dose settling in period, then I would go to her GP. They can then decide what to do for the best.