My girlfriend ex has recently started flirting and hitting on her, and this has started about a day or two ago. I don't know a lot of details yet. Cause I just found out that this just started happening, from her best friend, today. Should I confront her ex? And say stop flirting with my girlfriend? Or talk to my girlfriend about it? Or just wait a couple of days, and gather more information and see how it plays out?

It should be up to her to tell him to stop. If she's not, then she obviously enjoys the contact.

You should talk to her about this. It's not up to you to straighten it out with him, it is up to her.

Talk to your girlfriend. Ask her about what happened/is happening. Her 'best' friend may not know all the details or may be exaggerating. For all you know your girlfriend may have tried to to tell him to leave her alone but was trying not to make a scene.

She is not responsible for his actions. She is only responsible for her own.

Keep in mind that 'best' friends sometimes cause trouble for their own reasons. I find it interesting that a 'best' friend may have betrayed her/his best friend's trust as quickly as this one apparently did.

Don't listen to what anyone else has to say about your relationship. Advice or information from the "best friend" should be taken lightly. First of all this friend isn't much of a friend if they are trying to "snitch" on their friend who is your girlfriend. So I wouldn't take them to be a reliable source because they probably have a different motive. Second, you should talk to your girlfriend about it before jumping to conclusions. Even if it is true, she could just be trying to handle the situation herself so there's no drama.

If he's flirting with her, he obviously still has feelings for her. But he should move on, cause she moved on. Good Luck!!
:)