No. Toughen Up has no formaldehyde in line with CND policy not to use products that are deleterious to the nail plate.
Formaldehyde is very drying to the nail plate ... it removes moisture from it (why most hardeners contain it) which makes the plate go hard, and eventually very brittle. This is the reason I said not to use a top coat with F. in it if her plates are already in need of a strengthener.
Toughen Up works differently. It contains a chemical known as DMU which helps to naturally cross link the layers of keratin in the plate for extra strength and toughness.
We should all stay away from products that contain formaldehyde. It is this chemical that also turns nail plates yellow when people wear polishes that contain it non stop.
By the way. Formaldehyde is totally different from the formaldehyde Resin which does not have the same effect at all and is an ingredient that CND use in all enamels as well as Toughen Up as a stabilizer. Do not be confused it is quite a different animal, and is only a trace ingredient anyway,not an active ingredient.