There are a few ladies that I've known take up offers like this, and the one or two that I still keep up with seem to have done well with it. Myself though, I have never had the guts to actually go to work for a client, so to speak
The offers have been attractive, and somewhere I even have a regret or two for not taking the risk... but I had my reasons for declining, not the least of which being that I make a terrible employee and I'd hate to make one of my wonderful gentlemen fire me for riding a rolly chair through the halls and suggesting a "Topless Tuesday" policy at every project meeting.
A few points for you to consider - how well suited are you for this job? Are you certain that you have the motivation and skill set required to really excel at it? If you take the job, you'll be putting *two* sides of your reputation on the line, and you don't want to make any aspect of you look less than awesome or you risk losing this man as both a boyfriend and a boss. I'm sure he thinks the world of you, but there's fantasy and then there's reality as always.
Also, how close of a friend is he? Depending on your current relationship with him, are you sure that you want him to have access to your career life? Work is a pretty all encompassing part of people's lives these days, and many of us spend more time with our coworkers than our immediate family. Do you think you two would tire of each other after so much exposure? That answer may differ between you two, so it's extra important that you set clear expectations before you go into this. Lunchtime quickies are fun, but maybe not for everyone, every day if you're also being required to meet civie-style deadlines as well.
Lastly, how good is the job? Research the industry standard for the position offered, and make sure that you're getting what you deserve here... making sure to account for any extra duties added to the job description.