You thank her profusely. You have a small slice. You rave about how wonderful it is and how wonderful she is. What you don't do is eat so much that you feel bad.
Figure out how many calories are in it. Go for a run or workout. Better still why don't the two of you do a hike or workout together?
It's really hard when you are trying to lose weight, but one piece of homemade cake won't "ruin" what you have achieved and are still achieving.
Enjoy! Have fun! Just remember moderation.