'Nobody can sue you for broken ribs during CPR'
Perhaps not in the UK, but here in the US anyone can sue anyone for anything.
Including cracked ribs during CPR.
Now, they might not be able to successfully sue you, but they can file the suit, require you to hire counsel and go through pre-trial depositions and meetings, and rack up an easy ten grand in expenses, before it even goes to court. When and if it goes to court and the judge *maybe* throws it out, you might be able to counterclaim for your expenses and other damages. But you can have a couple of years (long trial calendars, very low priority for something like this) of worries and legal bills in the meantime. And if you were out of town on vacation at the time, or the suit is brought in a court district that not local to you...lots of travel expenses too.
Yes, legal matters can be that bad here. Yes, there are many attorneys who should be disbarred. Yes, there are lawsuits dismissed as "frivolous" and "bad faith" every day or week. But you still have grabbed a tar baby, and once you're caught up in it...there's no easy way to get free.