Old drain clogged with a lot of ceiling spackle

The man that fixed the ceiling over our tub cleaned his tools, pan, buckets in the little tiny bathroom sink. Now the drain is clogged and the area below is too small for my husband to get into. Because it was left empty so long the stuff in the sink has really hardened.

My question is – Is there some type of something that I can put in it that

1. won't cause more damage
2. won't cause an explosion
3. won't kill the man who lives in the basement.