There is no rule of disqualifying a surety merely on the basis of being a potential witness, indeed, such disqualification is an error in principle.
The practice has been created by the crown and mistakenly accepted by defence counsel, creating a "custom " of blindly rejecting such sureties.
E.K.,  O.J. No. 5852 (SCJ - bail review):
26 Finally, the learned Justice appears to reject proposed sureties on the basis that they are potential witnesses at trial. I agree with counsel for the applicant that to reject a surety on this basis without more amounts to error on principle. To agree to such a principle could lead to the unreasonable disqualification of a proposed surety who is suitable thereby frustrating the interim release prior to trial of individuals accused of criminal offences. Of course, some individuals may be unsuitable sureties based on the likelihood of being a witness. For example, the spouse who is the victim of domestic violence would not be a suitable surety in a case arising from domestic violence directed against her by an accused person. See Garry Trotter Law of Bail in Canada page 294.
27 The suitability of a surety is to be assessed on a case by case basis. In that assessment of suitability the likelihood of the surety being a witness is a factor that may be taken into account by a court. However, it was in error to disqualify a proposed surety on the basis of a rule of general application that any person who is a potential witness in the trial of an accused is therefore not suitable to act as a surety for that accused pending trial.
Brown,  O.J. No. 3946 (bail hearing - JP Dudar):
71 Finally, while the Crown did not raise this issue, the Court also concludes that there is a substantial possibility that one or both of the proposed sureties may be witnesses for the defence at the trial, in particular in respect of what may be raised as Charter challenges to the admissibility of the evidence from the search. Although there is no clear or absolute rule barring potential witnesses from serving as surety, there is a general disinclination to do so, on the basis that they join in interest with the accused and may colour their eventual evidence to strengthen the defence case. Being in the
constant presence of the accused may bring added pressure to do so.