My dad just turned 70 had heart surgery after mitral valve regurgitation severe from endocarditis, had bio valve, he is well now, full work up no cancers/cardio issues etc. after surgery has left bundle branch block. can he come gym and lift weights?

Yes, he can but his cardiologist and cardiac rehab facility should guide him through an exercise program.

Left bundle branch lock is less common after mitral surgery than aortic surgery. Whether he can return to full activity including lifting weight lifting will depend upon whether he had left ventricular impairment prior to surgery or after. Lifting weights is more stressful for the heart than walking or biking. Best advice, discuss fully with his cardiologist before proceeding.