Pepper spray is easily made at home. I told my friend how to make it and he now keeps a stay bottle of it in his car! You need to err on the side of caution - I think there are laws in various US states prohibiting the owning of certain weapons, and here in the UK there are various laws about carrying weapons, and pepper spray is illegal to own, carry or make.
This is not a good thing to post up on a public forum by the way. Yes, it is illegal to carry here, this will be treated as a firearm, comes under the Firearms act 1968 ((b)any weapon of whatever description designed or adapted for the discharge of any noxious liquid, gas or other thing; ) and if caught with it certainly faces a custodial sentence, the length of which will depend on what the charge is.. (most likely sections 16 and 19)
The provisions areā
(a)section 16 (possession of firearm with intent to injure);
(b)section 16A (possession of firearm with intent to cause fear of violence);
(c)section 17 (use of firearm to resist arrest);
(d)section 18 (carrying firearm with criminal intent);
(e)section 19 (carrying a firearm in a public place);
(f)section 20(1)(trespassing in a building with firearm).]
(2)The court shall impose an appropriate custodial sentence (or order for detention) for a term of at least the required minimum term (with or without a fine) unless the court is of the opinion that there are exceptional circumstances relating to the offence or to the offender which justify its not doing so.
Having it sat in the car means he will be unable to use it when outside his car, so the police may well decide he was carrying it not for self defence purposes but for something else.
I would tell him to get it out of his car and dispose of it responsibly
asap.