A letter: explaining a problem

I’ve had a problem recently. My bestfriend Lisa met a boy in the disco one weekend ago and he asked her out.
They've met three times and I think that she's fallen in love. I think he's a bit younger than her, but I'm not sure because she doesn't explain anything about him.
I'm worried because I haven't met this guy. I think my bestfriend should be more responsible with her relationship and I'd be not so worried if she introduced me to her new boyfriend George. But maybe I shouldn't interfere in their lives. Simply needs that my friend be happy. I usually trust Lisa, but she has been strange recently.
I'd like some advice because if I don't do something soon, we'll probably have a big argument.