can you cast wither bones when your own undead are in melee ?

[Norfleet]

Yes, the AI has no objections to inflicting friendly fire if you tell it to do so. In fact, it'll even preferentially target your own troops sometimes, since it isn't aware of deep and insightful tactics like "leading the target" or "missing to hit". Thus, it will aim directly at the target, and, naturally, miss and hit something else, probably something of yours.