It is important to consider all data and evidence when making your argument. Cherry Picking occurs when someone only considers the evidence that supports their claim, or excludes specific evidence that goes against it.
A common occurrence of Cherry Picking is using anecdotal evidence. Sharing anecdotal example can be helpful when framing your argument, but needs to be followed up with evidence that the anecdote applies to the general situation.
Mention some of the evidence that has been left out of their argument. Give them an opportunity to incorporate it or explain why it doesn't apply.
If they shared anecdotal evidence, ask them to explain why it applies to the general situation.