Hi,
You can witness the statement that they have provided, you do not need to get a JP to sign off on it. By them signing it they are saying the above statement is true and correct to the best of their knowledge, and with your signature it is saying that they have confirmed with you that this is their statement of events and their signature.
On a side note, this might be more applicable for placement in the general section of questions next time if you need any more answers.
Regards,
Linse.