(Just to say this is not a usual MIL thread. Although my MIL is a PITA in a myriad of ways, this is fairly lighthearted although I actually feel kind of sorry for her).
So. MIL (let’s call her June) goes to two bridge clubs, Monday night and Tuesday night. On a Monday night, she gets a lift from her friend who we will call Graham. Graham is, apparently, always moaning about something and I think they have nice little quarrels in the car but, of course, June being June obviously thinks nothing of it.
Anyway.
Yesterday afternoon, Graham rang June and said to her terribly sorry but (someone - friend of Graham ) had tested positive for the old ‘rona, he was isolating and unfortunately wouldn’t be able to drive her to bridge
BIL was at June’s and, feeling that June shouldn’t miss out, offered to drive her.
Well - and I think you can all see where this is going - who do you suppose was there already when June arrive?
It was, of course, that treacherous bastard Graham.
Cue a very awkward evening where June and Graham were pointedly ignoring each other.
Now, my DMIL is not one to be snubbed. Oh no. When she got home she was RAGING. She did, perhaps because of the late hour, decide to sleep on her rage.
The next day - today - she got to work. She rang EVERY SINGLE person in the bridge club individually to inform them of what a two faced lying shitbag Graham was.
After that, she then rang every other mutual connection of theirs. The contractor she recommended to him? Got a call. Her hairdresser? Got a call. The gardener they share (and his dad?) Got a call.
So it’s safe to say that Graham might find himself something of a social pariah come next Monday.
Now I know how MN feel about CFs and lifts generally. I don’t think MIL is a CF, I think Graham lives up the road and offered the arrangement (and, besides, it’s about a 3 minute drive but MIL can’t walk more than a few steps unaided so wouldn’t be able to get there (she has a car but has developed a bit of a fear for it, she’s in her 80s.) I suspect the reason Graham lied is because she drives him up the wall and he wanted out of the arrangement. But it was also a pretty stupid lie and obviously quite embarrassing for her - but bad enough for the nuclear reaction? 
YABU - June is a classic CF, Graham obviously had had enough of her and is perfectly within his rights to make any excuse he wants to get out of the arrangement and June is being vindictive and OTT
YANBU - Graham was being a dick to an elderly lady, he got caught out of a stupid lie when he should have just been honest if he didn’t want to drive her and June is perfectly within her rights to seek petty revenge