I have a client who has 3 domain names all pointing to her website homepage.
The homepage targets keywords relating to her main product.

She has another page - lets call it page2 - in her site, whose name consists of keywords of a second product.
Is it worth having a domain name with her second product's keywords which redirects to page2?

As long as the domain is relevant this sounds alright. The most important point is to ensure your domains won't confuse customers ("Why are exampleproduct.com and myproductexample the same???"). Search engines attempt to demote sites that are purely designed to funnel traffic to a page as spammers try to use this tactic. That being said, they try not to punish legitimate business trying to channel customers to their new products. Your client may be better off using subdomains so their customers see both the product line and the client's site. An example could be: productline.customerscompany.com/newitem

If the extra domain name she is using is relevant to the keywords she is targeting it will work. But if the domain is not relevant as per the keywords on second page then there is no reason for using a separate name domain name..

You can work on sub domains

Thanks everyone for the replies. Yes,the domain name is very relevant. One of the main reasons for purchasing the domain was to stop a rival company doing so, as this has happened before, but from your answers it looks like it might also be useful when pointing to the relevant page.

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