The best approach may be medical therapy which may improve the heart function enough to make surgery an option. Make sure the cath films are reviewed by a surgeon with alot of experience before excluding surgery as a possibility.
I had a similar experience 4 years ago and few years younger than your grandmother. I had had a "silent" heart attack and hospitalized in ICU for 3 days with congested heart failure, severe mitral valve regurgitation, enlarged left ventricle, totally blocked LAD artery (remains blocked but there is a natural bypass with collateral vessels), RCA 98% blocked and stented, and 72% blocked LCX (not stented) and an EF below 29% (weak contractions to pump). Recovery was quick and problem free and today I feel fine, heart size is normal and EF is 59%, and MVR is moderate. My exercise tolerance is good (treadmill fast walk (4.0 mph for 1/2 hour...7 METS).
For a perspective, a heart attack (MI) usually kills heart cells and pumping contractions are impaired. Sometimes the heart cells are stunned or hybernating and revitalized with treatment. Also, outcome depends on the degree of damage and location.
The option, without open heart surgery, is to open the occluded arteries and stent and supply the oxygen deficit area with a good supply of blood.
Subsequently, medication can reduce the heart's workload and with rest the relief can return the heart to functionality.