Activation at a later time

A friend of mine has a dell n5050 laptop and would like to upgrade to windows 10. He did in the past and the upgrade didn't last long before it (crashed - his words not mine, I wasn't there), anyway he is back on windows 7 now. I have just checked on the dell website and it seems his laptop model still isn't compatible with windows 10. My question is, if he performs the upgrade to 10 and then rolls back to 7 now (for his digital entitlement activation), will the Microsoft servers still recognise his activation on 10 after the free upgrade period is over, when possibly his laptop is compatible with windows 10. Or will the activation servers see he has reverted back to 7 and cancel his activation? I assume the only way to get digital entitlement is to do an in place upgrade or to use the get windows 10 app.

................will the Microsoft servers still recognise his activation on 10 after the free upgrade period is over....................

Yes, absolutely.
Once he has the digital entitlement, it is good for life of the computer. It will not be revoked even if the user has gone back to a previous operating system. It is still good.
He can reinstall Windows 10 any time, and as often as he wants, in the same computer.
That one year free upgrade restriction is no longer applicable to him.

..................... I assume the only way to get digital entitlement is to do an in place upgrade or to use the get windows 10 app...............

No. You can do Clean Install without going through the In-place Upgrade install or the "get windows 10 app".

I know his computer came with windows 7 starter, now he has windows 7 pro or whatever it is called, when he originally upgraded he got windows 10 pro, and he wants pro again if he can upgrade. Now how does activation work for 10 if he wants pro and not home with a clean install or do you mean upgrade install his 7 to 10, roll back to 7, upgrade with a clean install to 10 pro because he will have digital entitlement then. He does not have his new 7 product key, and I think he will choose to not loose anything, do an upgrade install - for the benefit of other users out there that have this potential question. Thanks for your other help.

Thank you David