my enemy has no cards in BF1. I have 1 with 7 might. in my turrn i play Bone Skewer, and my enemy reacts with Riposte. He can counter my spell?

Yes, Riposte can counter Bone Skewer.

Riposte just says "Choose a friendly unit and a spell. Counter that spell" — it doesn't require the spell to target or choose anything itself. Bone Skewer is a spell on the chain, so it's a legal target for Riposte as long as your opponent has a friendly unit to choose (FAQ 453: "a spell" means any spell).

The detail that Bone Skewer doesn't "choose" a unit until resolution (it only chooses a battlefield when played) matters for cards like Not So Fast, but not for Riposte — Riposte counters any spell outright. If it resolves, Bone Skewer is countered, does nothing, and goes to the trash (Rule 425.1.a–b).