If you've never worked a wedding before, I'd highly recommend suggesting that you'd be willing to work with them as a second shooter, but are not particularly comfortable being the sole photographer for the wedding as you lack experience working weddings. Then work with them to try to find a photographer who does have primary shooter experience and see if they would be willing to let you second shooter it as part of the referral.

The main key about weddings is that you need to be far, FAR more aware of what is going on and have very few, if any, re-dos. This means you need to be sure of yourself, familiar with how weddings flow, able to shoot on a pretty strenuous time table and no your technique cold so that you can focus on capturing the event rather than fiddling with your gear.

It isn't a good time to be learning on the fly without having someone there that is sure to catch anything you miss. You may end up doing fine and cover it great, but you could also make beginner mistakes and permanently cost them photos of key moments of their wedding. It simply isn't worth the risk if you can avoid it.

As for how to explain it to the client. Explain that while you are comfortable taking photos in a controlled environment, a wedding is an environment beyond your control and you would really prefer having someone there who is used to working in such an environment to cover you if/when mistakes happen.