Most of the people infected with this virus go symptom free for decades.
I am one such example. Married, have chidren into their teens, before I found about this virus. I am diagnosed as a carrier.
The only way to know whether your friend is a carrier is to get her tested as bberry suggested.
If she had antibodies 20 years back, most likely the HBsAg result will be negative (implying she does not have the virus now). Is it possible HBsAg is positive now ? May be, but that is rare and only way to find out is to get her tested.
They told her that she must have had HepB before. Even though she has had no problems in 20 years can she be a carrier?
if you test postive for antibodies then you are immuned for life . some people have less or more antibodies than others but all are considere immuned. for a low count of antibodies , some choose to get a boost shot which is not a bad idea.
if your "someone" had antibodies 20 years ago then they are cosidered to be cleared and set for life
as for you NOT needing the other two shots it might be the wrong idea the gave you.. medicine says 3 shots and the did not come up with the number out of the blue. there is a reason wht it is 3 shots.. so i would suggest you test yourself for antibodies again and see what level u stand at. if they told you u had antibodies then you were clear of this vurus at an earlier point in our life due to maybe a vaccination early in your life or by past clearance of the virus ( u were infecred, then fought it and cleared the virus on your own ,therefore, develop antibodies and become immune for life )
hope that explains it a bit