59 plate S4 gearbox sensor issue?

[Les]

Guys my pal’s son bought a 2009 S4 and a day later got this gearbox fault light. I believe this is a known fault from back then but does anyone have any advice on how to deal with it please?
Audi mentioned goodwill but this is looking unlikely as the age, partial service history and fact it’s been tuned aren’y Helping!
The guy he bought it from is denying any knowledge of the issue.
I haven’t been in here in a long time but still miss my B7 RS4’s.
Now in a F80 M3.........Sorry guys
Any good advice would be appreciated as the lad is crushed, looking at £2.5k to just fix the sensor or £5k to change the box (which they are recommending).