I'm going to lie to my friend's wife
Why will you lie to your friend's wife?
He asked me to tell her he was with me if she asked. I don't know what he's really planning on doing, but it's not hanging out with me.
You shouldn't lie to your friend's wife. It's wrong to lie without good reason, and that sounds like it's most likely not.
Maybe. I feel kind of guilty just thinking about it, but he is my friend.
Have you considered asking him why he wants you to lie to her? If his intentions are good, he shouldn't mind telling you. If they're not, you definitely shouldn't help him do something that will hurt her and potentially destroy his marriage.