I posted a link about a rape last night of a woman out jogging on a footpath at 9.00pm for a reason. Sadly it's not a one off. It happens. The big difference in the OP's case is that her husband isn't a rapist. The woman didn't know that.
You can't know anything about a random stranger, but you don't get to assault them for it!
I was raped in a park in the early hours of the morning when I was at university. It was traumatic and awful and not something I would ever wish on anyone ever.
But that does not give me the right to spray any guy with a chemical because he happens to run by me in a park!
Am I cautious when I am out by myself in deserted areas? Of course. Do I assault people who dare run past me? Of course not.
I'm fairly certain in English law that you don't need to wait to be attacked before you defend yourself but that you do need to argue that your belief of the threat was real and that your response was proportionate.I'm not convinced that she doesn't have a stateable case for both those issues.
I am not convinced she has any kind of case.
Section 3 of the Criminal Law Act 1967 states:
"A person may use such force as is reasonable in the circumstances in the prevention of crime, or in effecting or assisting in the lawful arrest of offenders or suspected offenders or of persons unlawfully at large."
In order for the attacker to argue her case she would have to show that she believed she was threatened by the sound of a man approaching at speed in a deserted towpath and her spraying him was because she felt imminent threat and it was the only reasonable response.
However
OP's DH was using a towpath that is well known as a running route, wearing running grear at an hour that might be early but is not beyond reasonable as a regular pre-work run.
He was running at speed but his attacker made no checks to verify his identity before she attacked him and therefore there is no way she could have possibly inferred he was intending to commit a crime in order for her attack to be proportionate.
If she had been concerned at the sound, it takes a second to look over your shoulder and see someone is clearly running towards you. If they then get towards you and make a sign that they intend to stop/touch you then that is the moment a threat becomes a reality and therefore attacking is proportionate. If the person running moves to run past and shows no sign of doing anything but overtaking, then attacking them with chemicals is a hugely disproportionate reaction.