I know this doesn't fix the initial problem, but it is actually very easy to destroy a relationship with just a dozen mean interactions. I've had to do it many times when switching between BFFs for one aspiration.
(In fact, while I used the mean interactions to "destroy" the relationship so , I temporarily switch to either of the "Deviance" aspirations so that those mean interactions count toward the 25 mean or mischievous interactions required in Step 1 of both Deviance aspirations.)
While it takes about 10-12 straight sim hours to build up to a BFF from stranger, it only takes a few minutes (or maybe 30 sim minutes) to lower the relationship meter to nothing. They will also forgive you pretty quickly if you want to bring their relationship back to a friendly status.
If you ever want to seriously destroy a relationship, just use a few mean interactions until the option to "Fight" comes up. Fighting with a sim completely fills the relationship bar with red immediately, and they are not just "disliked," but actually "despised."