The problem with the notion of telling people the truth, politely, is the idea that everyone or even most people respond well to honesty or want any sort of discussion about many situations.
OP seems like a nervous person to me, who hates confrontation. If her friend is at all confrontational then asking her to have a heart to heart is cruel and pointless.
The reality is that a lot of situations are better not to be discussed, just avoided, and that is a less harmful outcome in many situations.
That doesn't fit in with the soap opera therapy theme of recent times, but it's just as likely that OPs friend will be pissed off as she will be up for a chat about it all. We don't know these people.
There's a great phrase that so many should incorporate into their lives "Least said soonest mended".
So, OP, if YOU think she'll make your life difficult for asking her about what's going on, then don't do it. But if YOU think she will be up for an honest chat, then do that.
Obviously, you can't just not go, but the "friend" has not taken time off work, so it's absolutely fine to just cancel and give an excuse. Hurting literally nobody.
It's completely and totally fine to tell white lies at certain times. This might be one of those times.