to put my mother as a named driver to lower my insurance?

So long as she's legally allowed to drive and iher medical and mental state wouldn't prevent her (including informing DVLA as pp say) then it is not fraud or misrepresentation to have her as a named (but not primary) driver. My DH is a named driver of my car but last touched a steering wheel 16 years ago. In the event that circumstances arise that he might need to drive he legally could, and that's a valid reason to name him even if those circumstances never arise. It would be fraud and misrepresentation if she was declared as the main driver but that isn't what you are doing. Your colleague is wrong.

Fraud

I'm on my DS's insurance and it did bring his premium down. He's also on my car insurance because we swap cars when we need to. The question is, how often would your DM be driving your car? If not at all, then it's not worth the risk.

if you intended to put her as the main driver that is breaking law and would make your insurance invalid. if you add her as a second driver and she has a valid licence that is absolutely fine and acceptable

Generally adding an extra named driver is fine even if they are unlikely to drive, just doing it 'in case' is fine. However in your case she absolutely won't drive so it is dishonest and could cause problems. In any event once you declare the epilepsy it will be more expensive. If you don't declare the epilepsy then you will have a whiles heap of trouble.

Have you declared the epilepsy to the insurance company?

Her age may mean that it doesn’t lower your premium as expected. Costs can rise as drivers age as risk increases.

Your question is essentially, ‘Is fraud bad?’ Yes, it’s bad. Grow up.

• If you add someone only to get a cheaper rate, but they never actually drive the car, and you misrepresent their role (e.g., saying they’re a regular driver or the main driver when they’re not), insurers may treat that as insurance fraud (fronting).
• If discovered, consequences can include:
• Policy cancellation (you lose coverage).
• Claim denial (they refuse to pay out if you have an accident).
• Higher premiums in the future (you’ll be flagged as higher risk).
• Potential legal consequences in serious fraud cases.

Ordinarily, as a named 2nd driver, it's fine.

(a lot of young / inexperienced drivers often put their parents as the "Main" driver and themselves as 2nd "named" driver to get even lower premiums - That's known as "Fronting" and is illegal).

But...

With her Epilepsy - has she notified DVLA about that? - It's one of the medical conditions that people are legally required to declare.

(I know you said she doesn't drive now, so it's mostly irrelevant, but if you ever needed to make a claim and the insurance company decide to run some checks on both of your licences... Not necessarily relevant to your claim, but to see if they can find any excuses not to pay out for a claim... If her epilepsy hasn't been declared to DVLA, that would technically make her licence invalid, which may or may not then call into question the validity of your insurance policy... and "Hey Presto" - Gives the insurance company the ammo they're looking for to either deny the claim or come after you to pay them back)

Likewise on the insurance application form - I think there's a box on there to the effect of "Do you, or any named driver, have any medical conditions"

I'm being a bit overcautious there, but when you're dealing with insurance companies - you need to make sure everything is in order in case they do go over it with a fine tooth comb.