Does red hair need heat to lift?

My natural hair colour is red and it is quite a deep red. I've been going to the same hairdresser for years and years. I love her but she is so expensive.

I also have a friend who is a mobile hairdresser and a lot cheaper than where I currently go.

Anyway,I was speaking to my friend - the mobile hairdresser - and i asked her how she would get my hair to lift without heat (every hairdresser I've ever been to knows  that my hair is hard to lift without heat - I have 40 vol bleach highlights).  She said that she'd just leave the bleach on longer and that I'd probably get a better colour this way as the heat just shocks my hair into lightening. She also said the heat is damaging.

My hairdresser has a different opinion though. She says that I would probably find the bleach doesn't lift properly and it would end up more yellowy. She also said the amount of time she'd have to leave the bleach on without heat would be even worse for my hair than the heat.  I usually end up with the bleach on my hair for about 45 mins and that's under heat for most - if not all - of the time.

So just wondered who was right?