I completely agree with Suz T.
As an aside, it may be that she spends all her "awake" time at her daughter's because that way she doesn't feel like (or have to acknowledge?) that she is any sort of burden on your household, and therefore she doesn't feel any obligation to contribute to it. I'd be surprised if she weren't helping her daughter financially and preparing meals over there for her and her daughter. Seriously.
You and hubby need to talk and figure out some options that he (or you and he together) can sit down and discuss with MIL. It doesn't sound like you can really ask her to move out, since you said you really cannot afford the house you are in due to pay cuts. So she needs to start contributing financially.
My first item would be for hubby to let her know that you can no longer afford to furnish her with a cell phone. So if she wants to keep it, she needs to pay for it or it will be going away (I'm assuming that she has a line on your "family plan"?).
Also, she needs to contribute something towards the household utilities: electricity/gas, water, cable/satellite bill, etc. You can figure out an average for all of these and suggest a percentage of that average that she should give you guys monthly. She IS in fact using all these services, right? She bathes. She uses the bathroom. She watches TV. Her clothes are laundered. Right? (So for example: electric bill is $230/m, water bill is $40/m, TV is $50/m, so the total is $320/m. She could pay 15%, so about $50/month toward these household expenses).
You guys can similarly suggest an appropriate amount for her to contribute toward the household "grocery" bill. Her clothes are laundered there right? That uses laundry soap. Surely she uses shampoo and soap and toilet paper in the bathroom, right? Does she eat ANYTHING there? Breakfast? Her morning coffee? She creates trash that uses trash bags, right? I know some of this sounds petty, but it does all add up. And the point is more that her being there DOES have an impact on you and your family. That is what she doesn't see and your husband ignores.
That doesn't even touch on the fact that someone (you) has to go behind her to clean up (it's your house and you don't want it a mess, so SOMEONE has to do it). Suggest she pick one day a week to prepare dinner for the family. And suggest she choose one day/night a week that she runs a few loads of laundry and folds it, to help out.
Implement a household rule that no one sleeps on the sofa. That's the rule at our house (sort of). I do not allow guests who bunk here to sleep on the sofa. Period. My husband (due to weird work schedule) often doesn't sleep well at night. He will wake up and be unable to go back to sleep without the background TV noise, which he can't turn on in the bedroom without waking ME up. So he takes his pillow to the sofa at 3 a.m., on average, 3 nights out of 5. So I don't want anyone on it, in case he needs it.
And you and hubby really need to come to an agreement about the parenting of the 15 year old. Grandma needs to understand that while you know she loves him and are trying to "help" him, what the result is, is that she is ENABLING him to be irresponsible. That she needs to check with you guys before she unilaterally decides to do things "for" him (cleaning his room, taking him to school, buying him anything over $15, or whatever). In a few short years he will be a legal adult, so time is running out to "grow him up". It's time to back off of "helping" so much.
Say all this with love. You DO love her. But she is impacting your household in unintended ways. So your hubby needs to have a family meeting and discuss the reality so that you guys can all work toward an agreeable solution.