Need help migrating from 2007 macbook to 2015 macbook pro

I am helping a friend migrate their files and system info from their old 2007 macbook to a new 2015 macbook pro.

They had been backing their macbook up with time machine, but the new computer will not recognize the time machine backup because the operating system is too old.

So....I am thinking to directly connect the computers, booting the older computer in target mode.

Problem is new computer has thunderbolt, older computer has firewire (400).

I can buy a thunderbolt to firewire (800) adapter for $29, and then a firewire 800 to 400 for another $10.

This should get me connected. But will this connection support booting the older device in target mode so that I can use migration assistant to transfer files?

I hate to spend $40 if it won't work.

Or, would I be better off just getting a thunderbolt to ethernet adapter for the new Mac Pro, and then directly connecting the devices with an ethernet cable.

Still costs $29, but the thunderbolt to ethernet would probably be more useful than the thunderbolt to firewire 800, as she has no other firewire devices to connect.

Anybody have any advice or experience with a similar setup?

Thanks.