You could just e-mail the customer back to apologise that you're unavailable and leave it at that. Or you could recommend someone else. Some photographers don't like to recommend someone, as they think they'll be helping out a competitor. But the fact of the matter is you can't do it yourself, other photographers may well be competitors, and you may often be competing for the same work, and both trying to maximise your own slice of the pie, but this doesn't mean that you shouldn't recommend them.

1. Good customer service is prized, and if you recommend someone and the customer is happy with the recommendation they may well remember you for your good customer service
2. If the customer has approached you then chances are they liked what they saw in your previous work and were keen for you to work with them, on occasion they may have just scattergun approached every photographer they could find, but sometimes they did actually want you. You may not be working with them this time, but they're still your customer, keep them happy and they'll approach you again if they want you.
3. They may remember you and recommend back, especially if they're successful and frequently fully booked themselves.
4. Photographers are a close-knit community and by recommending someone you know you may in turn find out about how the job went which could be useful information.
5. By recommending someone you know will take the work seriously and deliver quality you're helping to ensure that the customer doesn't have their head turned by someone offering to do the job for a measly fee. Preventing potential customers from going to a photographer who will lower their expectations on both quality and price. If they do end up going to someone working for a pittance instead of you that may mean that in future they expect you to work for a similar amount.

I can't really think of any reasons not to recommend. If you can't do the job then they're going to go to another photographer either way so the argument that you're helping someone else to your detriment doesn't really hold. It can only benefit you if you keep yourself in their thoughts and involve yourself in seeing that their needs are met.

It is of course important to trust the photographer you're recommending. It reflects badly on you if you recommend someone and they do a botch job; and also if a mutual trust develops then you may find that they're more than willingly to pass multiples jobs they can't do onto you if you do the same for them, which is especially useful if you're a sole trader as sometimes for reasons outside your control you simply can't do a job you'd taken on. Illnesses, injuries and other disasters happen. A friendly, trustworthy colleague is more likely to save your ass than someone who doesn't know you from Adam.

I think my key philosophy, and one I hope you will take on, is one of seeing other photographers less as competitors not to be trusted, and the photography industry less as a zero-sum game. Sure they are competitors and sometimes you will need to try and impress upon a potential client that you feel you are the better person for their specific job, but this doesn't mean that they should be viewed as always working to undermine you, or that you should try and undermine them; and there are plenty of occasions where trusting each other a bit more can be to your mutual benefit. So try and play nice.