We had this problem with eldest dd's best friend last year - dd then being 5yrs old, quite 'easy-going' and amiable, she said her friend was preventing her from playing with others, and in the classroom if she tried to sit next to another friend, her bestie would physically stop her going. 
I had a word with the teacher about it. Normally I try and stay out of friendship bickers, but I knew that dd would've already tried to deal with it herself, to no avail, by the time she brought this up. I asked her if she'd like me to help, and she said yes.
So next day arranged to see the teacher after school - dd was expecting me to go in without her and sort it with the teacher myself, but I asked her to come with me. (She was a bit surprised but was fine otherwise). Reason being I wanted her to see and hear all that was said, so she was aware, and to feel like she had taken an active part in solving the problem.
The teacher was really supportive (she hadn't been aware) - and said she'd keep an eye on the situation, and also said she'd do some work with the class about being a good friend etc. Dd said she would let the teacher know if she has problems like that again. And I said I would help dd practice at home, how to handle certain situations more assertively. Dd was helped to understand that she wasn't getting her friend in trouble, because we knew she liked her and they were best friends - it was just that sometimes dd wanted to play with other friends and that it's fine to do so!
Anyway - it seems to be sorted. The teacher said 6mths later in end of year report how dd was much more able to say to her friends what she did/didn't want to do. They're still best friends a year later - but dd tells me they've worked out some sort of arrangement where they have certain days in when they play with other friends! Good luck OP