By the way, there's no need to catastrophise things. If relationships don't work out, that's a shame. But it doesn't follow that it will necessarily ruin people's lives. Relax.
It's a general rule of thumb but not some kind of iron law. Personally, I think the OP's 'friend' should go for it. It sounds like someone younger is probably better at unlocking those emotional defences this friend has built up.