What do I say when a friend wants me to be her alibi?

1st I really can't lie well at all. Second I believe in honesty and marriage. I've been there and understand how annoyed you can get with your mate, but marriage is not something to trample on.I have even hated my husband at moments in our lives. Still, wrong is wrong.I wouldn't dare judge her but I can't allow myself to get pulled into the problem either. I swear this is how I've lost so many friends. I don't want to lose her friendship, but I won't lie. Any helpful suggestions welcome. Thanks.