Not able to replace a forgotten password.

One of the site owners has forgotten his password and I tried to change it in the members section. When I replaced it and tried to log in, it said username or password incorrect. How can I change the password?

When you say site owners do you mean they have admin status? Have them try the forgot password function, it should mail them a new one.

They did that and still the password would not work. He is the site owner, I'm the admin. I tried to change it for him after he said that the forgot password function didn't work. I tried to change it in the admin area under members tab and his profile. There it has a spot for changing password but still doesn't change.

i am still little confuse about who can't login but try clearing cache and if nobody can login try replacing member.php file with a new one.

http://www.boonex.com/forums/topic/Reset-Admin-Password.htm

It's not the admin password he's trying to change. Did you try to login as him with the new password after you changed it in admin? If you can and he can't then the problem is on his end. 

What happened when he tried to use the forgot password?

That above post can change any password. Not just the admin. As long as you know the ID the method i used in that post can change anyones password.