Three reasons she cant: 1) medical MJ isnt legal in her state, so every transaction would be subject to possible criminal charges. 2) THC is hella more expensive than her subsidized opioids and shes on a fixed income, and 3) THC isnt sufficient to deal with all types of pain, just as opioids dont help with all types of pain.
Oddly, we both ended up with different chronic medical problems that include chronic pain. I use opioids and THC to control mine, but I live in a state where its completely legal and am a bit more financially secure. The opioids dont work on all my pain, at least not at strengths where Im still functional and taking a higher dose is scary. THC also doesnt work for all my pain unless I take enough to be loopy. Ive found, though, that taking a small dose of opioid and a small dose of THC actually works really well on most of my pain and I can still function almost normally. I also use higher doses of THC to get much better sleep (it works way better than all the prescriptions Ive been given over the years and has near zero side-effects). Better sleep also reduces the perception of pain.