On page 13 of the Beta rules, there is the classic "indirect grenade" diagram. The same basic diagram and rules as in the original ruleset.
I have two questions:
1: the nightmare is not in the commando's front arc. He can't throw the grenade without turning.
2: The rules really should allow sideways tossing of grenades, at a penalty of course. If you aren't picky about placement, a running commando shouldn't have too much trouble tossing a grenade into a room as he runs past.
3: Can the Nightmare target the commando with his Nachtmacher? (Does indirect grenade throwing work both ways?). I can imagine the nightmare would have enough visibility around the corner to give it a good try.