I'd suggest that she stop diluting the alcohol and mist it on full strength. She should at least give that a try.

I don't think that trying to mix alcohol with vegetable oil would work

soundgarden, mealy bugs infest also the surroundings- planters, walls and the like. Such areas also should be cleaned. As for the plant itself, she might remove the worst-infested parts. This will leave her a smaller plant to treat and will expose more of the pest to treatment. It sounds like a huge infestation and will require persistent work to bring it under control. Mealy bugs will migrate to the other plants so she should isolate the hibiscus if possible.

Mealy bugs are close relatives of Aphids and can be treated the same, spray with a sharp stream of water to knock them off the plant, repeating as necessary. There is no need to use Rubbing Alcohol (that can be harmful to plants) or any kind of oil. Since most all pests go after plants that are not as healthy as they should be check the soil these plants are growing in to be sure it is good and healthy and can grow a strong and healthy plant that will better withstand an insect invasion.