A whole foods plant based Diet preferably with no added oil is the best diet. Key marker is to bring LDL level to 70mg/dl or lower. Test LP(a) cholesterol as well. If it’s very high, might need take PCSK9 treatment on top of statin Atorvastatin & Ezetimibe.

Good luck to ur dad. But if it is multi vessel blockages, at 56, a Bypass is recommended. Especially if the blockages involves the LM, LAD. Lcx & Rca perhaps can just do balloon if blockage under 60% & stenting if blockages exceeds 75%. To control Coronary Heart Disease, Plant base low oil diet, healthy lifestyle + statin is effective treatments.