How can I know if someone I bought a greyhound ticket for ever boarded the bus? I'm not sure how to find out. I looked on greyhound's website but I couldn't find any help there.
There is no easy way to do this at the moment other than contacting the person themself. Greyhound still uses paper ticketing so the paper ticket its self is the record they keep -- it's not always easy to find where that is. If you ordered a will call ticket for this person you may be able to call up the station and see if they can tell you if anyone ever picked it up.
If you got the person a refundable ticket and want a refund if they didn't use it, you'll have to get them to give you the ticket back. If it was a refundable will call ticket that they never actually picked up, Greyhound may be able to help you via the phone or at the station it was on will call.