If you have any concerns about a witness’s availability for a hearing, or their ability to testify in a credible manner, you can have them sign and date a written statement or, even better, swear an affidavit. In BC, there is a long list of individuals who can help you swear an affidavit, but the two most common examples are notary publics and practicing lawyers. Obtaining an affidavit may cost some money, but it can also carry more weight than an unsworn statement in the eyes of an arbitrator.
If a witness is unable or unwilling to attend a hearing or sign a written statement, you can try to explain to the arbitrator what you think that witness would have said. This type of hearsay evidence – or testimony about what someone else saw or heard – may be accepted by the arbitrator, though it will likely be given less weight and consideration.