Although it is valid to have an apostrophe in an e-mail address, it is not good practice.
They'll run into other problems with an e-mail address like that because most software protects against them going into the database.

My advice would be to tell them to use an alternate e-mail address (everybody has more than one right?)

You could also change the validation directly in the core, but I wouldn't recommend it.

Yikes, that is long. I'll change the core, perhaps remove validation for that field. The site is an small intranet, and the 'alternate' email solution won't work. I know it would be a lot more difficult to change the email address from a company level, from  O'Brien@whereever.com to OBrien@whereever.com

Thanks