Would not be surprised if it is the ebola that killed him.
Interestingly, the article does not mention whether he was feeling ill prior to demise.
Ebola and heart attack have quite different symptoms.
What people forget is that people die of other things in Liberia, just like everywhere else.
In fact, somewhere around 50,000 people died in 2014 in Liberia from various causes other than Ebola. About 3000 died of Ebola.
“Hiking” in a tropical environment would be pretty stressful for a 58 year old guy.