No the soccer club is not liable.
They had nothing to do with the theft and public liability only covers parties that are injured at an event of the club due to the clubs negligence.
If the party you gave the car too was negligent and left their personal items in the change room they are responsible.
The change rooms are generally accessed by anyone and leaving their personal items in there, they have been negligent.
If the change rooms were to be locked and that was a policy of the club, and he relied on this he can sue them for negligence but the public liability insurance would not cover the claim.
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