Not quite sure this is an emergency and that mold had to be on there for a while. So firstly, don't panic!
There is a black-mold concern that can be a health issue. Recommend masking up and gloves before any scrubbing away at mold.
I can't tell the material the mold is on. Some materials are much more difficult to get mold removed from than others. I'm not sure I understand "lid can't be removed" but ok. Get some good thick ply plastic from a big box home store. Seal up the tank with plastic and duct tape, realize there's some risk here. Might want to go lighter chemicals first - peroxide, diluted bleach , dawn soap - just in case some get into tank the damage will be less than commercial "fungal-cides". There are plethora of products available appropriate to most materials if it's persistent and if she's really worried about it. Also increase the air flow inside the canopy. Drill a hole or two should be sufficient but you can supplement with small fans. These are available at aquarium supply places. Essentially a small computer fan will do. With good air flow and some mild treatments I would think it could be brought under control easily if not outright eliminated.