Unwilling to “share” a friend
Before You Begin
Pause and breathe. Even if this shouldn't be happening... accept that it is and embrace it with composure and calmness.
Calmness is the only way forward because without calm, there is no possibility for receptiveness and connection. Your child won't give to you any more than they see from you.
Never forget that a strong connection is fundamental, as there will be no correction in them if there is no connection with you.
The Core Problem
Possessiveness of the friend, plus the insecurity that they will lose the friend to someone else.
What Will Make It Worse
Any comments that invalidate their feelings and/or give them more reason to think that their worries will come true.
What The Child Needs
1. To look at it from their friend's perspective: Help the child understand how their possessiveness might make the friend feel. Your child wouldn't want to be under the control of another friend, so have your child use this reasoning to allow themselves to end their possessive approach to the friendship.
2. To prove themselves wrong: Encourage your child to "do a test" and organize group play activities. Use these experiences to help the child see that their fears didn't come true. The friend can play in a group with them and others, and even have play dates with others, yet they are still coming back wanting to play with your child.
How To Have The Conversation With Your Child
Find a private space where the child feels comfortable to talk without distractions.
Start with the observation that you have noticed your child gets uncomfortable and even mad if their friend is playing with someone else.
Assure your child that you hate to see them feel this way, and ask what it is that is bothering them in those moments. Listen actively and without interrupting so the child feels heard.
Next, talk through the above points under "What The Child Needs".
Try to help the child arrive at the conclusion that the only way to resolve these negative feelings of theirs is to have an open mind.
In the following days/weeks act on point 2 and take every opportunity to praise their efforts to be ok with sharing the friend.