Give/lend a helping hand
Definition 1 of 1
Pro Tip 1/3Offering Support
Used when offering to help someone with a task or problem they're facing. 
Noticing her struggle with the report, he lent a helping hand.
Pro Tip 2/3Voluntary Action
Implies that the help is given willingly without expecting something in return. 
He lent a helping hand at the charity event without being asked.
Pro Tip 3/3Momentary Assistance
The phrase often suggests a temporary act of assistance, not a long-term commitment. 
During the event setup, many volunteers gave a helping hand.