Regardless of age difference or not, this is an important life lesson. Friends WILL want to play with other friends, and they're entitled to do so. Friends are not your possession. A girl may see another group of girls doing something more fun, or feel closer to those girls than your granddaughter and whenever they're around, she may decide to ditch her. It is sad, but it happens, it's life. In the adult world, it happens when dating too, if your partner finds another person to be more fun or compatible, you risk losing that person.
Best thing to do is to learn to play by yourself and be creative. She can play with some dolls, a tricycle, a coloring book, or decorate something, do arts and crafts, play with you, sing, dance...you cannot expect people to want to be around you and entertaining you 24-7, so it's good to learn to have imagination and find ways to keep entertained until someone comes along that is willing to play.
I used to have this problem growing up all the time, and I used to get jealous and accuse kids of taking my friend away from me calling them friend stealers, until my mom explained to me that my friends are free people and can play with whoever they want, whether I liked it or not. I had to learn ways to play alone until someone else came along, or I'd play a sport or join a group of kids doing something and whenever I was bored or tired, I'd leave the group.
I wouldn't forbid Bella from playing with your granddaughter, just explain to her that Bella wants to share herself and her time with other kids. Tell her to cherish the time she had with Bella and learn to share her with other girls who enjoy her company and don't get too upset about it.