Back Sweetening a Lambic

I made a framboise for my wife 1-1/2 years ago. Just kegged it yesterday. I like it, but my wife isn't sold on it yet (she said it's too sour, and not very sweet). I need to add sugar to it, so I picked up some lactose today. The question is...how much should I add to a 4.5 gallon batch? And will the Brett eat it? I have it under refrigeration now, so I think that should limit most activity. Or am I mistaken?

You will have to add a known amount to a glass and go from there. I use a shot glass and add by the gram. Then I extrapolate by the amount and add that to the keg. You can also take that extrapolation by .9 and hit the sweet spot. Pardon the pun. Na, don't bother.

Since it's kegged you could just add a simple syrup of water and table sugar. The cold will prevent brett or bacterial activity (for the most part). However it's probably easiest to just add sugar in the glass.

Did she think she was going to get a lindeman's-like lambic?