This is probably going to sound like whining but 

What if your best friend (or at least a very dear friend) becomes really really friendly with your mutual enemy? 

Best friend - A  

Mutual enemy - B 

Mutual friend - C

A year ago, A would probably be complaining with you about B, something along these lines: 

" she's so annoying and look how fake she is. She doesn't have any friends so she's talking to C hoping that C would be her friend urgh." 

Now, A would be doing this when B comes around: 

"Hey, B. (Insert nickname) How're you? Are you free for lunch today?" 

*A notices C with her earphones connected to B's iPod* 

"Oh you're listening to her music. Huh, her music taste is good anyways. Do you like that song, C?" 

And when you walk by, they all ignore you 

And now, B's birthday's around the corner so everyone's planning for her birthday and because you're 'friendly' with her they (B's group of friends) decided to let you in with the discussion for the party: 

*A opens a group, adds loads of people in it, sends a message and adds you because she forgot* 

Random friend 1: so we're going to do this and that and this

Random friend 2: yeah sure why not 

you: oh hey guys, sorry but I'm not free 

A: WHaT the hell 

you: I mean I'm doing something with B (it just happens you're doing a workshop together and you're the only ones in your class joining that) so we won't be free until that time 

A: okay then 

*talking talking and more talking* 

Random friend 1: Yep I can go on Saturday 

Random friend 2: I'm free too 

Random friend 3: Wait let me check 

Random friend 3: Yeah I'm okay also 

You: so maybe we should do it on that date? 

Random friend 1: sure, why not. What about A? 

*twenty minutes later* 

A: Sorry for late reply, but let's do this *talks about something totally irrelevant to mentioned points* 

All the others agree 

A: *starts laying out plan, talks about the lunch.* so 1 and 2 should distract A when they're buying lunch 

(totally ignores you for the rest of the conversation) 

what do you do? 

(Oh btw forgot to mention nobody did anything for you for your birthday except for A who sent you a long message and C who bought you sth)