I have a friend who is turning into a bit of a Wendy.
She lives near me and didn't really know anyone when she moved here. I made an effort to be-friend her and also introduced her to my group of friends. All started well and we would have nights out and a good laugh.
Over time she started to arrange to do things with my closest friend. She would invite her round and not ask me. She would invite her shopping and not ask me. My closest friend and I have known each other for 30 years, so I was determined Wendy wasn't going to spoil that, but I was also happy she did things with others.
I have just made sure that I do things with both Wendy and my closest friend, but make lots of time just to talk to and still do things on my own with my closest friend, as I have always done.
Wendy can railroad people into doing things they perhaps wouldn't do - she can be quite forceful, without realising it, although she does mean well. I'm quite assertive, so just politely tell her to sod off and laugh. She does railroad my closest friend into stuff though, but she needs to deal with that herself.
I've never mentioned what I suspect Wendy is up to with my closest friend, as I don't want to sound possessive or jealous, but there's not a chance in hell I will allow Wendy to break a 30 year friendship.