Nope, it doesn't. Simply gas is a leaner fuel than petrol and that's the reason, why the light comes on ;)
It says that something is wrong to me,a decent LPG conversion should have a system that allows for this and prevents the EM light from coming on... As I said previously, there are plenty of poor or even dangerous conversions about. I would question why he is ditching the car 12 months after forking out a hill of cash for a conversion...
I've run my 300 sport cross on LPG for almost two years now. The EM light came on shortly after the conversion - running too lean. It's a quck fiddle with the injector trim (it has 6 aditional injectors for the LPG and these need to open and close at as near the exact timings of the petrol injectors) via the installer and it's never come on again.
...cant fault LPG - slight loss of power (like having a passenger in the car) but it's offset by the 55.9p per litre (filled it up this morning for £31.99) which means the car hardly see the inside lane as the fuel's so cheap... might as well drive fast.
Good luck
Jack