I had some happy hypomanic times in the few years between my first, ammmmmmmmazzzzzzzzzing summer of hypomania and the begining of knowing something was very, very wrong with me (dysphoric mania). And during those times a few times someone who either tease me or say that I acted hypomanic and I blew it off every time as just my personality, how I was handling stress, or even "just having fun with/for the kids at camp" where I was a counselor. There was no way I was going to hear that message until I was ready which was several years later and I had to realize that my moods were what was wrong with me.

There's nothing wrong with saying something in a carin way but unfortunately you probably won't be heard until the other person is ready. On the plus side you've shown willingness to talk and if they are bipolar they may someday really appreciate that openness.