Girlfriend spoils her son (6 years old) and I can't take his bratty behavior. Advice?

That pretty much sums it up. She buys him toys/treats/lets him have his way on the regular but when he doesn't get what he wants, he whines and/or throws a fit and she ends up giving in. He's 6 but feels he can boss her around, doesn't use polite words (please/thank you/etc.) unless pushed, whines/acts up/acts bratty/throws a tantrum if he doesn't get what he wants. When I refused to play with him because of his bossy, spoiled attitude, he threw a fit and told his mom "I hate you" and other mean things. He also tries to manipulate her with stories of what his other mom lets him do (less school work, etc.). They may feel they are in competition for his affection, which complicate things, but I don't think either one should act as though they are. I was raised to show respect and behave and some of the behavior I've seen in this child would never have been tolerated by my own parents.

I can give you lots of specific examples if needed but you get the idea. I can't take his disrespectful, entitled, bossy attitude, it is souring my positive feelings for him. Any recommendations on how to talk with my girlfriend about this? We've talked before and she knows his behavior at times is a problem, but I'm seeing a connection with her lack of discipline and spoiling him that I don't think she sees. I mean, I googled "spoiled children" or something and the 7 signs to look for - he had every single one and I have seen that she has displayed the behavior of the spoiling parent.

I am the girlfriend and don't have children of my own. I don't think this means I couldn't raise a well-behaved child if I had my own child though, nor that I don't understand children or how to discipline.

More details to give you a better picture: we are 10 months in and live together. Neither of us believes in the spanking method or withholding food. I believe the child should lose privileges when he misbehaves (take away toys, no show time, etc.). Although my girlfriend would verbally agree with me, I have never seen this happen with him. He acts badly the whole day and she tries to appease him with toys and fun stuff instead of not putting up with the bad behavior.

I do love the little boy (he's not always this way obviously) and for the most part we have a great relationship. He acted very spoiled and bratty recently and I really cannot stand it when children give lip to their parents, are rude, bossy, and throw tantrums trying to control their parents' behavior, and say rude and mean things to them.

Any advice for my situation (being the girlfriend and living with them)? Thanks.