These kinds of software licenses are often non-transferrable meaning that if you buy the game for yourself, technically you shouldn't be letting other people play your copy.
Is that a silly rule? Maybe. But the whole point of having Steam-linking and GOG-linking is to protect FAF from being accused of facilitating copyright violations.
I have some good news, though: "Supreme Commander: Forged Alliance" is on sale NOW at Steam for just $2.59
So if your friend has a Steam account, he can buy the game for himself for just $2.59
The offer ends in 20 hours, so: act fast