Use 'commiserate' when sharing someone's feelings of sadness, not just acknowledging them. He commiserated with her, feeling the weight of her grief.
Pro Tip 2/3
Situational Usage
'Commiserate' is often used in situations of loss or disappointment, rather than minor inconveniences. The team commiserated over their defeat in the finals.
Pro Tip 3/3
Not For Joy
Don't use 'commiserate' for happy occasions; it's reserved for times of sorrow or difficulty. We commiserated with him after his business failed.