If your husband is not prepared to defend this then there are only a few option.
1. Husband pays into the scam, is marked as a "mug" for future reference, your family is £100 out of pocket and hopefully that is the end of the matter.
2. You husband transfers liability to you as advised above and you can then defend the PCN.
3. Do nothing and risk getting a CCJ by default.
You appear to much too sensible for option #3 and you are made of sterner stuff than your husband. So, ideally, option #2 is your best bet.
If this ever reaches a court, it is easy to defend. No solicitor is required and there are some solicitors on here who will be giving advice.
The "threatening letters" are just debt collector letters and you need to understand that they are powerless to do anything. They are a third party to the contract that has allegedly been breached. They usually work on a no-win, no-fee basis by using scary words such as "CCJ" and "bailiff". You can safely ignore them.
You defence would rely on many points and there is a template defence which you only have to edit one or two paragraphs. You would include Jopson v Homegard and if you need some seductive bedtime reading, have a look at it here:
https://www.dropbox.com/s/9ezhkj6epu66l1r/JOPSON-V-HOMEGUARD-2906J-Approved.pdf?dl=0It is an appeal judgment and so is persuasive on the lower court. The main point in that judgment is the fact that loading and unloading is not considered to be parking. There may be other cases that could be referenced but we can cross that bridge when we come to it.
Defending this would e a valuable life lesson. Unfortunately, our education system does nothing to teach anyone some basic facts that would assist them in understanding civil law. It never ceases to amaze me how many people have absolutely no understanding of what a CCJ is or how it can affect you. Considering most people will come across something like this at some point in their lives, the fact that nothing is taught about it, even at a basic level (or at least it wasn't when I was a lad) is astounding.
You will receive assistance and advice should this progress to a claim. It is nothing to be feared and and is just a process that needs to be understood.