Do the job for your normal chargeable rate.
You are a professional (electrician, like me). You’ve bought tools, trained, paid your NICEIC/NAPIT/SCAM fees. You’re insured. You can do the job and he/she can’t.
The advice I’ve read on here that you have given to others is always good, so you are bringing quality and professionalism that he otherwise may not get (unless I did it of course!).
He’s getting your Sunday; time away from the girlfriend which you won’t get back.
It will also set expectations for the next job he wants done. He’ll tell his mates that you only charge £x and that’s what he, and they will expect you to charge when they want you to bail them out of a bit of electrical diy they’ve made a mess of.
You’ll end up regretting doing it because as every tradesman knows, the jobs you do as a favour are the ones that come back to bite.
Don’t sell your services for less than they’re worth unless you are doing it for VERY close family (parents/children) or because the person benefitting is a worthy cause. (I do some work FOC for a lady who’s got MS).
</here endeth the lesson>
Rich.