Drama.
It'll probably just be a misunderstanding, like her and her coworkers are there too or something; though it is interesting that she wouldn't have told him, since volleyball is a much more participative activity than sitting quietly and talking >> if I had a significant other and I was in this situation, then I probably wouldn't bother to say I was going to sit and talk a little about romance advice, but I would definitely mention if I was going to go play volleyball in a bikini.
Its 100% going to be just a coincidence/misunderstanding drama bait/tease and all will be well, but I don't understand why it needs to happen in the first place - It's so easy to text someone, saying "hey Im going to do X with Y at Z for a bit, see you tonight"