Or, more to the point, when her nipples are not erect.

I did what too many people do not do--use one of the many internet search engines.

Here is a quote from the first search result:

If you have always wanted to wear a silver nipple ring but were too scared to have a piercing then these are the perfect pieces of erotic jewellery for you.

So easy to wear just twist on around your erect nipples. Wind up or down to suit your nipple size.

The silver is very soft so you can open to fit most sizes. Gently pull the erect nipples through, or get your partner to hold the ring in their mouth against their teeth and gently suck through. Then adjust by squeezing them gently for a secure fit. They do not fall off if secured properly. Should they lose their shape at all just wind around a pen or cylinder type object and press back to its natural state.

Can we pierce nipples??

Yes.

Lips
Tongue
Cheek
Nose
Labia
Penis
Clitoris
...and no doubt other places.