Well, there was such a mod once, but it's old and outdated now and I don't really know that I'd recommend messing with it. The whole (base game) BFF thing did undergo at least one change -- I know at first, you couldn't change your BFF at all. Then they made it so you could choose a new BFF, though you'd lose the old one, still allowing only one at any given time. As far as I know, that's still the way of things now.
The old mod can be found at: modthesims.info/d/539718 if you want to have a look at it. I don't think I would actually recommend trying to use it at this point. (It hasn't been updated since Dec. 2014 and there've been a couple pretty big patches since then.) Definitely make sure to back up any save games and such first, if you do decide to try it at all.
At this point, I think the old mod's usefulness may be pretty much limited to being a starting point for anyone who might want to make a newer, up-to-date version.
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