According to the Wilton chart, that cake should feed 140 guests. I have found that the more flavors of cupcakes you have, the more people will eat. If you have cake, plus a couple of flavors of cupcakes, you should be able to plan for one per person. Any more than that, you'll want to think about planning 1.5 per guest. People will generally only eat one piece of cake, but sometimes will eat more than 1 flavor of cupcake.
So you will want to make anywhere from 160 cupcakes to 240 cupcakes, again depending on flavors. Because she will have a cake as well, she will probably want the cupcakes to match the cake in color or theme, which could limit your options. (For example, if I made a cake and cupcakes for a bride who wanted me to color match both, I couldn't offer her chocolate peanut butter or cookie dough cupcakes since they can't be colored, etc)
I would probably try to encourage her to make it as simple as possible, not only for budget reasons (unless she isn't concerned about that) but for presentation. If she was just having cupcakes, more flavors wouldn't seem nearly as overwhelming to guests. For a cake and cupcakes, I'd stick to as few flavors as possible.