Hi again. If your friend has a 2500 credit limit run of the mill card, a Wells fargo visa, he doesn't travel on business, then one credit card is perfectly acceptable.
Just my opinion, based on my experience.....No, you cannot attain a 750-800 score with one basic credit card. Here's why.
My credit score is only affected when I improve on it [new cards, credit increases on existing cards, lower utilization ratio overall, etc] or damage it [late pays, high utilization ratio(using over 25% of total credit available, I believe)]; normal usage 'seems' to have no +/- effect on my score.
That said, over the yrs, I have acquired new cards solely for the purpose of increasing my total available credit which has increased my score, thereby allowing me to qualify for lower interest cards. Also, mortgage and auto loans represent a portion of your score.
While neither is necessary or mandatory, having either or both in your history contributes to how high your score goes and how quickly it gets there. fyi, I was a 580 about 10 yrs ago, i'm currently sub-750 with good credit mgmt.