Slowly losing empathy for my friend

I'll call her Becca. She likes this boy. I'll call him DK.

Becca is unhealthily obsessed with him. They used to date but he broke up with her a few months ago for another girl who he thought was prettier. Becca cried to me about it several times after that and was devastated. Since then, DK has been with a ton of other girls and is always talking to multiple girls at a time. My brother is actually in DK's athletics class and hears him talking about these girls to his friends in the locker room and sometimes he sees him show the other guys naked pictures of them. Disgusting. My brother emailed the coach about it twice and got no reply even though he's emailed him before and gotten quick responses. I find that VERY weird that he's suddenly getting nothing back.

Becca is still not over him. She talks about him everytime I'm with her and it's like she has no personality anymore. Like he took it from her. He used to fat shame her in front of people at lunch and I would be like "um wtf" and then there's Becca who just tries defending him and saying that he's just worried about her and I shouldn't be so judgey towards him. ???? Bestie what??? I mean shes like 10 lbs overweight at most and he would say stuff like "getting a little chubby aren't you?" Or "maybe you should buy some new clothes that won't show your rolls as much" ROLLS?? WHAT ROLLS?

DK has started playing with Becca's feelings again and is talking about possibly getting back together.

[Side note: when they were dating he would threaten to leave her like every week for a variety of reasons. Examples: "You're too fat", "You don't dress cute like the other girls", "You don't do 'girlfriend' things" etc. Apparently to him 'girlfriend' things are sending him pics, making out in the hallway (ew), and letting him touch her whenever he wants and not complaining about any of it. Who the hell raised this kid?]

He's stringing her along and using her for entertainment but she just won't realize it. She refuses. It's like she's knowingly living in a fantasy world where he loves and cares about her. Like deadass she'll ask me for advice and then ignore all of it. She wants me to be on board with her getting back with DK but it's just not going to happen. I'm concerned that it's going to negatively effect her long term if she does. He's scum. I mean, he started paying attention to her again like not even 2 weeks ago and has already asked her for pics twice. Does he know that porn is free? If I were her I would've kicked him in the dick so hard, it would be fucking inverted.

(Btw I know he hates my guts because he kept trying to get Becca to drop me as a friend. She didn't though so I win 😘)

She keeps making things worse for herself literally any chance she gets. It's insane. Sis needs help and I'm willing to give it but I can't make her take it. I honestly don't know what to do or say anymore. This whole ordeal with her is just pathetic to watch. I feel embarrassed for her because she just lets him step all over her. Btw not to brag or anything but I'm seemingly the only person who gives a fuck because no one else is trying to help or stand up for her when DK talks down to her. They just talk shit and cringe. I feel like I'm becoming more and more like them every day because it's hard to feel bad for someone who has every opportunity to better themselves and their situation but chooses not to.

Life is hard and I know it wouldn't be easy for Becca to give up on her little DK-crush-fantasy thing but she has the support from me and her own free will. She needs to start living life and I want to help her so bad. She has so much potential and she's so pretty and could easily find another boy to obsess over if she wanted.

It's to the point where I'M stressed out.