Top oven/grill not working on a Whirlpool double oven

Hi guys,

I sold my old (pretty nice) kitchen to my mate for a very cheap price, including lots of units, dishwasher, hobb, fridge and double oven.

With the double oven, he’s fitted it himself and said that the top grill/oven won’t work at all, but the bottom one will. What is the likelehood he’s just got it wired up wrong (he wired it in himself)? I tested it before I took it out and am pretty certain it was working fine, but now his girlfriend has text me asking for some money back ¬_¬ (ie almost half of what they paid in total)

Thanks