it's all oil, but depending on how much he put in.... Urgh. I'd get it out ASAP, flush, and probably run through again. It probably won't screw seals up, but I'd personally want to flush it through as far as possible. You're talking a 0.25 to 0.5l topup in a system which contains <4l, so it's potentially 20% of the engine oil.
- Bret