I prefer to do the sides first, mostly because I don't like pieced borders, and this saves on a little bit of fabric length.
I'm confused about your statement "Is it easier for a beginner to do the sides first and extend them past the top and bottom border". If you do the sides first, the top and bottom will extend over the sides.
I think it's more important to measure your borders carefully, so that you don't get wavy borders. If you friend hasn't discussed this with you, come back and ask!
Good Luck with you quilting journey. It's a wonderful trip.