You could tell them in a text
I start by warning them after they sit down that I don't have good news. You then mention the person, and what their condition is. The hardest part is being prepared for how they react to what you're telling them. Of course, everybody reacts differently so the next part is up to your discretion whether to console them or leave them be
I would say tell them early, because they'll have time to work out their feelings over the vacation without further external stress, but telling them at the end kind of ruins the whole experience anyways. The best thing to do is get them alone and be straight forward with them. The only thing you can do past that point is either tell them, or dance around the subject, which just makes the impact worse the longer it goes on. If you put it off, they're most likely to have the realisation that this has been happening behind their back the entire time, which isn't any help either