To answer your question in terms of "value" I would look at the rate the room was otherwise available for and divide in half. That's what the miles were worth on that night.

As to the moral issue everybody does stuff differently so I am not judging you. For ME if I give points/miles to a friend it's without strings. If they buy me a nice dinner, or something, that's great but for me life is too short to worry about every penny like that. Plus, it's something nice I can do for my friend. Maybe they will repay one day and maybe they won't. In fact, I have given Marriott points and airline miles to friends many times. By the way, I pay for all my own travel myself not some corporate conglomerate. Again, that's ME and not judging you for how you operate. I should also add if I was going to have a friend give me money, which I would not do, I would make sure they knew of this before all the plans were made. I would not want to surprise them with it after the fact. Feels a bit tacky to ask for money but especially to surprise them after the fact. Good luck to you.