If I were that dad I either would have waited to clean the guns or called you and asked if it would be a problem with you. If you had been ok with it and your kid asked a question about it while he saw him cleaning, the dad may have given him some information about guns and how they are not toys and to make sure if he ever finds one not to touch it and so on.
I believe kids should be taught about guns but should not have access to them until they are older and understand just how dangerous they can be.

