2 domains pointing to one site

A member of my forum has just informed me that he has decided to close down his own site as he is unable to continue to support it.

However he still has some time left on his domain registration so he has configured a redirect from his domain to one of my forums as this forums subject is related to the domain name. I am somewhat flattered by this, however I am concerned that this may have an impact on SEO.

If you enter the URL of the now deceased site, it immediately and non transparently redirects to the sub forum on my site. For example:

http://www.wibble.com

redirects to

http://www.mysite.com/wibble-forum/

Is this a problem or not?

as long as it redirects, its fine. the problem would be if wibble.com LOADED yoursite.com/wibble-forum/'s content