It's not going to work as your (wife's) bike is currently setup. Ultegra is a 24mm spindle while a Rotor 3D+ is a 30mm spindle. Your mechanic is right in that a new Bottom Bracket is required to fit the Rotor 3D+ as the current one in there is for 24mm spindles.
If you wanted true plug and play then get a Rotor 3D24 (or original Rotor 3D) which is the exact same fitment as an Ultegra crankset. I can't speak as to if a special BB removal tool is required for Cervelo as I don't own one or dealt with one so just don't know if your mechanic is correct or not.
No adapter will work to make a bottom bracket accept a bigger than originally designed spindle. If your bike has shims to reduce the diameter to fit the Ultegra crankset then if that is the case you can have those removed and they most likely will be the correct 30mm diameter you'd require. But again, I don't know what is on your specific bike being talked about.
If you wanted true plug and play then get a Rotor 3D24 (or original Rotor 3D) which is the exact same fitment as an Ultegra crankset. I can't speak as to if a special BB removal tool is required for Cervelo as I don't own one or dealt with one so just don't know if your mechanic is correct or not.
No adapter will work to make a bottom bracket accept a bigger than originally designed spindle. If your bike has shims to reduce the diameter to fit the Ultegra crankset then if that is the case you can have those removed and they most likely will be the correct 30mm diameter you'd require. But again, I don't know what is on your specific bike being talked about.