Did I have an hallucination?

Sounds like a hallucination.
Check your meds & dosage.

….or could be a ghost sighting

It could be any number of things. There might have been someone and she just didn’t see him.

@Marina i concur.

There are several options, and it is not wise to jump to that conclusion. It may have been a hallucination, your friend may not have seen him as @Marina said, or she may have been intentionally messing with your mind (people love to play on my colourblindness, telling me that grass is red etc.). If it happens again then see someone about it, but until then don’t worry too much.

Your friend may not have noticed the person you saw or may have not really looked carefully.

If you have further instances of seeing or hearing things others you trust do not, see your doctor for a proper evaluation.

Hallucinations I had, long ago, were very colorful and not weird guys in hallways. More like trails and drippings trees, etc. So, no probably not.