I just remembered that DD1 had a few times - not many - where she'd pretend to play with someone else. Not a friend exactly, she just pretended that someone else was there.
DH didn't like it as it wasn't real. The same as when she'd crawl around barking like a dog. He used to tell her to knock it off. I'd have to sit & explain to him that its normal & healthy to do those things & that I never want him telling her to stop again as I didn't want to discourage her imagination.
To me thats the most important thing a child has & something they need as an adult. Not quite so out there as an adult I hope, but I want them to be able to think on the spot & I think that helps.

Don't worry too much as it'll probably help her when there are problems with friends at school.