Online travel insurance - but you will have trouble covering a 78 year old, and it will be expensive.

This question has come up on the forum before - it depends also on what is available from their home country - some countries have 'reciprocal medical care' agreements, where you can be covered automatically - your visitor should check with their health/travel government authority in their home country.

I'm Australian and I was in the medicare office one day and an older Greek person who is resident in Australia was having a loud conversation - they had just spent $4000 on medical treatment in Greece, whilst on holidays, not realising that they were covered under reciprocal medical agreements and could have activated it if they had contacted the Australian embassy in Greece soon as they had a problem...oops !