What could a co-signer do to prevent or stop a foreclosure ?

Pay the arrears.

Yes as a co signer you have promised to pay the bill if the original signer does not.

So what you are required to do is pay the amount due to stop the foreclosure ( as you promised to do when you signed for the other person)

If not, of course this will reflect on your credit also and they can also come after you for a judgement to get their money.