It sounds far too close to me.

Whilst the train from Manchester is reasonably reliable, delays do happen. Likewise with the tube and HEX. And to arrive in Euston at 07:30 you'll be having to wake up pretty early so there is the chance of missing the alarm clock.

If you're on a flexible ticket then go for it and if you miss the flight then just get transferred to a later one. Otherwise how about celebrating your friends birthday at lunchtime?

The other option is to fly MAN-LHR-USA with all the flights on one ticket so that you are protected in the case of a delay.