it totally depends. if someone has heart problem, then if it's possible that you can spare them the dreaded news, then do so. but if it's something he or she has to know then i guess go ahead and spill it. but take all things to consider. maybe catch him/her in a good mood. or in his/her healthiest form. never rattle him/her. do it gently. and be sure to say it in person. at least you'd be there to assist him/her in case something happens.