The Unwritten Rules of Friendship is a very good book and well worth a read. Lots of people on here recommend it.
Its horrible not being able to be there and help them navigate the minefield of playground interaction isn't it. I know when my ds was going through this I just wanted to be able to go and help, but ultimately I guess all we can do is guide from the sidelines and they have to find their own way. I think that's where the book comes in, as its a way of enabling parents to help/empower their children without actually having to be 'there' to resolve the problem in a hands-on way.
I guess she'd be in reception year at age 5? I think its a difficult year for a lot of children, as they are finding their feet in the school environment and at the same time, learning to negotiate the playground. I think most children have good days and bad days and many of them have days where they can't find anyone to play with at playtime and get upset about it.
If it helps, things tend to improve by the time they get into year 1, as by then they've usually identified the group of children they feel most comfortable with and tend to gravitate into little teams a lot more.
My ds2 regularly got upset about playtime/lunchtime in reception year, but now in year 1 he has identified the children he gets on best with and knows where to look for them in the playground, so can go straight over rather than wandering around looking for someone to play with.
Anyway, I'm rambling. I definitely think reading the book is worth a try though.