Considering Suggestion isn't technically a charm, but that those who are immune to charms are immune to its effects, does it logically follow that a Warlock could turn suggestion back on the caster? Would the Warlock get to instead tell the caster what they want them to do, would they do what the enemy initially tried to suggest to the caster, or would they just have the charmed condition?
This question/answer pair talks about Fey Ancestry and what happens if you are charmed, but not of suggestion, nor how a Warlock could retaliate to this spell being cast on them.
Beginning at 10th level, your patron teaches you how to turn the mind-affecting magic of your enemies against them. You are immune to being charmed, and when another creature attempts to charm you, you can use your reaction to attempt to turn the charm back on that creature. The creature must succeed on a Wisdom saving throw against your warlock spell save DC or be charmed by you for 1 minute or until the creature takes any damage.