Before she replaces her catalyst, there might be another reason why she has less power with the check engine light on. EEC systems use adaptive logic using input data from the various sensors, including the O2 sensor where the best air/fuel mix is calculated dynamically. When a fault is detected - like the 02 reading in the cracked manifold - the EEC might switch to a “Limited Hardware Operation Strategy” (HLOS) where the entire EEC system and A/F mix is run on fixed values (all calculations cease). This will cause greatly diminished performance. I would suspect she has a BIG crack in that manifold, perhaps causing this condition. She should check the condition of the catalyst before she replaces it - but get that manifold replaced and you might as well do the 02 sensor as well.