This is legal
Help is an Action you can take. Ready lets you prepare an Action you can take to be used with your Reaction when a specific trigger is met.
Exerpt from PHB pg. 175, Working Together:
A character can only provide help if the task is one that he or she could attempt alone.
Here's the portion on Help, PHB pg. 192:
You can lend your aid to another creature in the
completion of a task. When you take the Help action,
the creature you aid gains advantage on the next ability
check it makes to perform the task you are helping with,
provided that it makes the check before the start of
your next turn.
Alternatively, you can aid a friendly creature in
attacking a creature within 5 feet of you. You feint,
distract the target, or in some other way team up to
make your ally’s attack more effective. If your ally
attacks the target before your next turn, the first attack
roll is made with advantage.
A familiar can't take the Attack action, but nothing is preventing them from using the Help action.
PHB pg. 240:
A familiar can’t attack, but it can take other actions as normal.
Help is an Action that can be taken, just like Dash, Dodge, and Hide are.