I have the same issue. My child sim met someone at school. i invited her over and the played. They also went to the park and play etc. Then when i clicked on the sim i had the option become bff forever so i did that, but my box isnt ticked off. i do have other boxes done and the box for good friend also went up to 2 with this sim.
The only reason i can think of is that in the description it says that you'll find the option under Friendly, but with me i already saw the option when i just clicked the sim.
i saved my game and closed it and then started it again because i thought it was a glitch...But it didnt help.
Now what?
