Ask to Break Up With...

Their romance bar must be completely full and they both have to be in a flirty mood for the option to appear
*Edit* If you still don't see the option, you could always play as the other sim and while playing as them have them break up with their boyfriend

The option to ask other sims to break up comes from The Sims 4 Get Together expansion pack. It is located under the Mean interactions (because you are mean to the sim you want that sim to break up with). This can be done by any sim, but if the relationship is low, it won't work.
Here's what you should click to find this interaction: Mean.../Other Sims.../Ask to Break Up with...
If the sim is on the same lot, you'll see them break up immediately after the action is complete. If not, you'll recieve a text message later saying that sims have broken up.
This is actually part of influencing NPCs' relationship feature. The interactions are located under Other Sims... category: Talking other sims up and hooking sims up are part of friendly category while asking sims to break up and trash talking are located under mean category. Additionally, you can ask other sims about other sims to find out the relationship between them.
Or have I accidentally stumbled in The Sims 3 discussion?

"EgonVM;c-16614334" wrote:
The option to ask other sims to break up comes from The Sims 4 Get Together expansion pack. It is located under the Mean interactions (because you are mean to the sim you want that sim to break up with). This can be done by any sim, but if the relationship is low, it won't work.
Here's what you should click to find this interaction: Mean.../Other Sims.../Ask to Break Up with...
If the sim is on the same lot, you'll see them break up immediately after the action is complete. If not, you'll recieve a text message later saying that sims have broken up.
This is actually part of influencing NPCs' relationship feature. The interactions are located under Other Sims... category: Talking other sims up and hooking sims up are part of friendly category while asking sims to break up and trash talking are located under mean category. Additionally, you can ask other sims about other sims to find out the relationship between them.
Or have I accidentally stumbled in The Sims 3 discussion?

You're in the right section! You're also completely correct on these options.

Thanks, all. I'm going to keep working on it. I do have Get Together and I know I've done this before, it just doesn't seem to be showing up as an option anywhere for me. I have tried while both sims were in a flirty mood, however neither their friendship nor their relationship bars are full. That may be my problem. :)

I'll keep trying!

"LaurenLoser16;c-16616608" wrote:
Thanks, all. I'm going to keep working on it. I do have Get Together and I know I've done this before, it just doesn't seem to be showing up as an option anywhere for me. I have tried while both sims were in a flirty mood, however neither their friendship nor their relationship bars are full. That may be my problem. :)

I'll keep trying!

Not even in mean section?
I may have to open the game to see if the option is still there for me. If I find it under mean/other sims section and you don't, I'd suggest repairing the game thorugh Origin. Be right back!
Update. I found the Break Up With... under the mean/other sims section. So it's still in the game.