I use the cruet that you can get free with a package of Good Seasons Italian salad dressing mix (even if I'm not using the mix) to get the oil/vinegar proportions right. The only "secret ingredient" your friend had was probably simply the brands of oil and vinegar she used, which can only be trial and error for you if she isn't around to ask. The flavors vary a lot.
I once had an aunt quiz me mercilessly about how I made mine, and I just told her the truth ... I use Good Seasons Zesty, extra virgin olive oil, and at the time it was balsamic vinegar (not the super expensive stuff, the less expensive). It was funny because I was in her house and saw that she was using distilled vinegar and corn oil and regular (not zesty) seasoning packet. Huh? NONE of the ingredients were the same as mine! Sometimes people ask you for a recipe then accuse you of holding out on them!
OFten it isn't a "secret" ingredient, it is just using the same ingredients.
When I'm not "cheating" and using a package, I use my favorite olive oil and vinegar, a clove of garlic (quite often, if you like a milder garlic flavor, you can just rub the clove over your salad bowl, then toss it). Pinch of salt, grind of pepper. A nice touch is a dash of worchestershire sauce.