I was on facebook and i noticed my girlfriend liked her friends post and commented on it. My girlfriend post read "Wow i just see a picture of my ex and i literally bought him that whole outfit on his profile pic. I will never fall for a scrub again or buy another guy clothes.". My girlfriend liked it and left a comment saying "lol right".
Does this mean my girlfriend has done the same mistake in the past?
It just means she agrees with her friend and thinks it might be appropriate or okay for her to friend to say that. She is "supporting" her friends decision. It doesn't mean she can relate. You don't have to go through the same thing to feel sympathy.
Not necessarily. She probably felt in that moment it's smarter to avoid spending that kind of money on someone (even if she doesn't really believe that) and is just showing support to her friend. She's just showing agreement it doesn't have to mean she's been through it. But if that's something you'd truly like to know, asking her is the best option. No one here can guess it for you.