Choosing between two differently but equally well qualified candidates for the internship vacancy coming up in my office. I emailed their four referees (two each) on Monday, and have had the following responses:

Candidate A:
First referee is an EU official who has changed job and email address, but was at least still hooked up to their old system, and replied with a positive testimonial.
Second referee is a university professor who has not replied to my email (which on inspection was sent to the general departmental address as given by Candidate A). I googled for other email addresses this morning and forwarded Monday’s email to them. I also called the department but he only works there on Tuesdays.

Candidate B:
First referee is an ambassador based in Geneva, who has not replied to my email (which on inspection was sent to the general embassy address, as given by Candidate B). I called the embassy this morning and they promised that the ambassador would call back today. My hopes are not high as it is a busy week in Geneva.
Second referee is a university professor who has not replied to my email (which on inspection was sent to a personal address as given by Candidate B). I googled for a university address and forwarded Monday’s email to it this morning. No telephone number on the university’s website.

I think it’s quite likely that I won’t hear back from any of the other three; this is a frenetic time of year for university professors and for ambassadors in Geneva to be asked to give testimonials for people who worked with them for a few months some time ago. In that case I will go for candidate A who at least had one person willing to speak to their abilities. But I feel I should let candidate B know that I was unable to contact their referees and consequently offered the position to another candidate. I will also advise candidate A to update their referee coordinates.

Any thoughts on how to do that tactfully?