So first, white discharge is usually normal. As you've said, women's discharge can vary a lot during the month, and as women go through different changes in life. She could have a slight yeast infection or something, or if she started or changed methods of birth control, that could cause it.
Obviously, there is more going on here, and I don't know if you're asking us if it's semen or just discharge. There's no way for us to know that.
Does it matter, though? She could be having sex with this other guy and using condoms, and you wouldn't have physical evidence. It's not as if this is your only clue that something inappropriate is happening.
I'd say the panties alone wouldn't be nearly enough evidence of something, and could be anything, even with the going/staying out and relationship/friendship with the other guy.
The going/staying out and other guy are separate from the panties, and are things that need to be addressed, even if the panties didn't exist.
I'd suggest counseling, and if she won't go, go by yourself.