Total is $7100, she makes $3000, so wouldn't it decrease to $4100, not $3100??
Also, since the written expenses are $5700 but they don't have $1300 leftover every month (or maybe it's $200, after the $1100 TFSA you mention), clearly those expenses aren't accurate.
The investment accounts would only throw off about $5600/yr (with the 4% rule applied). Assuming they will have $4100 if your mom quits, they need $1600/month, so now, they don't have enough. However, if they get rid of the $1100 investment and cut that missing $200, it's close. But the other issue is that your dad's disability goes away. What does their SSI situation look like?
To me, it seems like it she can't quit yet, but perhaps could cut down to 2 days, unless they want to trim the budget.