First, let's talk about the situation. You were visiting a AN ELDERLY FRIEND ("senior friend" is not too clear) in a hospital, and he GOT up from his chair to greet you. And you said to him,"hey, please be seated."
In this situation, where you were talking to someone (much) older than you, and someone who was not well-- and therefore someone you wanted to be very polite to--your choice of words wasn't quite right. It would have been better to say,"Oh, won't you have a seat, please?" or "Oh, you don't need to get up."
Remember, that the command form "DO something" is ALWAYS a command form, and it is generally not polite to give commands to someone, especially if you're talking UP to someone older or someone in a higher position than you. Using this command form, even with the addition of "please" is considered VERY rude by most English speakers, so please work on learning how to express your "commonds" as polite requests.
