Actually Micheal's answer is not *quite* correct. She can sign in with her AppleID and either *purchase* things or play her music which has been copied to the computer. This will not affect your Match account.
You will be locked out of your account for 90 days if she :
- Redownloads previously purchased music
- Turns on auto-download
- Turns on Match
It all has to do with the confusing distinction between "authorization" and "association", explained here:
Unfortunately, with iTunes 11, as soon as she signs into iTunes, all her previous purchases will show up as cloud content, even if she doesn't have match, so it will be very tempting to click on one and thereby lock you out of Match (she'll get a warning about this, but we're used to ingoring warnings). In her account, I strongly advice you to do View->hide items in the cloud to prevent this from happening.
Many thanks for the advice. To be honest all previous purchases are in the main library and saved on a back up drive as the nano she owns is an older generation soot was just the occasional song downloaded. I think going forward we will stick to just one account to save any errors or issues.