See, I just looked up that scene on YouTube, and best I can tell you don't actually tell her that Solas just dumped Lavellan. You can only tell her the truth about Vallaslin and she makes it clear she doesn't know what happened after the Inquisitor went off with Solas. You're expecting to show empathy for something she doesn't know happened.
I never said she knew that Solas broke it off. What she did know was that my character was looking and sounding sad (the option was the Sad option), and as a friend instead of laughing at her, mocking her, and making fart noises, she should have been able to offer some emotional support. But she didn't. So she is not a friend. If I opened up to someone about something I had that many strong personal feelings about, and was clearly feeling down, and they treated me like Sera did? I would at the very least tell them off. Maybe I'd punch them. It's like, "My friend is telling me she's sad. LOSER. HAHAHAHAHAHAHAHAHA YOU SUCK LOSER YOU ALWAYS DID YOUR PEOPLE ARE GARBAGE HAHAHAHAHAHAHA." And that's a seriously ugly way to be. That's not fun, light-hearted ribbing. There's a time for light-hearted ribbing, and that time is not when your friend is clearly down in the dumps about something.
Others' opinions about her lack of an end goal to her actions, and her unwillingness to take responsibility for her mistakes, also apply. But I could overlook that; for a clever person she doesn't seem very intelligent in some ways. She's rather childish.
She doesn't need to know Solas broke it off in order to know it's a bad idea to laugh. Lavellan has been raised on the idea that she is part of the last free elves who has not been broken by their history. They are taught that the dalish will never again submit to humanity or accept servitude/slavery. To learn that the mark of her heritage of her face was actually a slave brand has to hurt.
Anyone with even a shred of empathy would realise laughing at what must have been a very painful revelation is a horrible thing to do. She might dislike the dalish but if she cared about the Inquisitor at all she would have reacted differently.
Edit: I could add that Josefine seems to realise the situation just fine with LESS details and change the subject. Sera just laugh.
Agreed. Sera doesn't just laugh, either. She laughs, she makes noises, and she gloats about how basically the Dalish are a bunch of losers.
I'm actually beginning to agree with some others' opinions that Sera is an anti-elven racist. I can get not agreeing with the Dalish. What I can't comprehend is failing to respect them as people--and never being able to grow enough to understand that just because their culture is different, and their beliefs, doesn't mean they are somehow better, worse, snobbier, etc. than you are. There's a reason the Dalish are so insular, and it involves people like Sera--and people like Gaspard, who also is the first to blame elves for all the problems, when in actuality he's about to cause a host of problems himself.