I was thinking about this thread while going through my morning routine with my plants....
First thought was that pests on indoor plants is a fact of life -- at least for me. You deal with them as they appear. That's why it's important to inspect your plants often, and incorporating this as a habit or part of your routine helps in early detection/discovery.
I thoroughly mist all my indoor plants every morning, and most of them get turned every other day for even/fair coverage with the filtered water misting as well as light. This turning is critical because often I will discover heavier infestation in the blind spot/back of the plant when the area comes into view. You could also do this when you water or when you turn on/off supplemental lights -- I think using automatic watering and timer for lights are convenient, but you lose valuable one-on-one time with your plants that could lead to other issues.
Scale insect infestation is not difficult to deal with in early stages but devastating when discovered too late. I've lost a plant earlier in the winter due to late discovery and perhaps too severe treatment on the weakened plant. I've tossed them away on finding infested plants more often than I care to admit -- mostly due to sour memories of days and weeks of effort to eradicate without success.
However, scale insects are not very mobile and rarely infests multiple plants in my experience. Usually, discovery is limited to one plant. They only pass from one plant to another when they are in contact. (Although I have suspected ants of moving them from one plant to another since they do feed on the honeydew from scale insects, just like aphids. More than once, I have discovered scale insects on plants due to unusual ant activity on the plants.)
So if your friend/house mate has serious scale insect infestation on her plants, she would probably welcome your offer to help eradicating them. Since this is a new arrangement, perhaps you might gift her with new plants and gently suggest she sacrifice the scale infested plants. You could probably bring in your own plants and quarantine them in a separate location.
Personally I don't use poisons -- anything-
cides -- even on ornamental plants and particularly not inside the house. I don't use chemical fertilizers --nothing blue or green and no slow release pellets or spikes. (I put live earthworms in every pot
) This kind of plant care philosophy is something that you may want to discuss before too long. You may also want to discuss how you would share plant care -- each care for own plants except to plant sit when one is away? Take turns?
I've started there for the time being, and have it in my own space. It's not much, but it'll give me something green to tend to.