If the dimmer is not working properly then you should replace it. It might just become unsafe.
Simplest is to replace it with an on/off switch, that is within most DIY capabilities so long as you take care to properly isolate the power (safest is to switch off whole home, otherwise you need some means to check you
really have isolated that light). Unless they have a strong preference, I would just use a plain MK switch such as this one:
Order online at Screwfix.com. FREE next day delivery available, free collection in 1 minute.
www.screwfix.com
But please do not get the "MK essentials" range, they are rubbish!
If you are looking to replace the dimmer with another one you do get ones that are optimised for LED lighting. Unfortunately that is not as simple or reliable as filament lamps were. I have used these to reasonably good effect along with dimmable lamps:
Choose Varilight V-Pro dimmers to control dimmable LED lighting.
www.varilight.co.uk