If you fit a decat or sports cat to an FRS normally within 2 weeks or so the light will come on

You will need to either fit a MIL eliminator or tbh the better option would be for your mate to invest in either a Bluefin map with the decat file or get the similar version of dreamscience

If he does not have an uprated map on the car and just fitted a decat then he may actually find the car will run very rich & will actually lose power This was proved by a lad on the RSOC who fitted a decat with no other mods at all put his car on the rollers and lost about 20bhp off standard and car was overfuelling very badly