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If your client is hosting and operating the site, I can't see there being a problem with this. However, if your name is connected to the site hosting agreement in any way, then I agree with the original comment made by Dante7237.
I believe the rules vary a little depending if the site is hosted in the UK or the USA, but legally you may be responsible for the comments posted in that forum if the forum was used for any illegal activity.
For example, if your client used it to encourage a patent violation (it doesn't have to be porn) and you were unable to moderate the forum when someone complained, then as overall site webmaster I'm pretty sure you could be held liable for the consequences.
You would be better off signing a non-disclosure agreement than losing control of the forum IMHO.
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