To be dubious about Green Flag advice?

[BracedlnEndIessJanuary]

I am hoping that IABU then I shall worry less. My boot was left ajar - did not notice, my fault entirely. Car battery totally dead this morning.
Green Flag came out, it is a Bank hol so great service. But the mechanic basically jump started it then went off.
He said the alternator is fine. Leave it on idle for 30 minutes as if you drive it and stall, it will not start up again. After 30 minutes you can drive it if you want but it is a myth that driving high speeds charge it faster or better than idling, new cars these days etc
But it is not a new car, it is 9 years old. The battery was dead as a dodo. I have done what he said, idled for 40 then drove for 30 with no radio, air, lights etc but am still panicking it will not start in the morning.
Halfords do do a home service for new batteries but not on bank holiday.
Is the mechanic right?
Last time I had this many moons ago, the RAC put it on a speedy battery charge and waited 20 minutes. Do they not do this anymore?

Hi OP. The mechanic would have tested the alternator was working alongside the battery when they came out and the advice they have given you is quite reasonable. I might be tempted though to start up a few more times just to be sure over the course of the evening. If it is safe to do so leave it to go for ten mins or so each time.

Thank you. I shall try it in a bit. Cheers x

It’s no different from DH jump starting my car for me so I could drive to work. It’s only a 25 min journey & my car started fine at home time, although I did brace myself for it not to so understand how you feel. Teen left his interior light on and drained his battery so DH has jump started his too before now. The alternator charges the battery as you drive and, as there was a reason for your flat battery other than an alternator fault, there’s no reason to think it won’t now be charged so try not to worry.