How “underweight” is underweight? I ask this because being underweight is very subjective. If a person is 6’0″ and weighs 140 lbs. (sorry I live in the USA, we don’t use the metric system here) I’d say that person is underweight. If a person is 5’7″ and weighs 140 lbs., that person might not be THAT underweight. So this is a pretty important fact as it pertains to low blood pressure.
Low blood pressure can be caused by many things. Some are good, some are bad. People who are lean (not underweight), athletic, etc. will tend to have a low blood pressure and pulse simply because their arteries and blood vessels have very little resistance or plaque build up. Their hearts, which is a muscle, is strong and can push blood with no problem and very little resistance.
But the flip side of low blood pressure is the bad parts. Low blood pressure can also be caused by a bleed internally. This is a “worst case scenario” type situation. This would be associated with a higher resting pulse rate, and possibly a tender or painful abdomen, and dark or “tar” like stool. Again, this is usually associated with a combination of other medical problems.
So if your friend is not complaining of the other medical issues I mentioned, maybe his being underweight is what is causing his low blood pressure. And for the record, low blood pressure is not that big of a deal….unless it is being caused by the medical conditions I mentioned. One thing I forgot to mention, and this is important. If you friend DOES have some sort of bleeding issue AND is taking aspirin, he needs to stop taking it immediately. Aspirin is an anti-coagulant, which prevents blood clotting.
A physical could answer all the questions. Hope that helped, and didn’t cause you unnecesary fright.