There is no proper answer to something like this. It depends heavily on the person's standards of beauty, what their exact feelings of themselves are, and if there is even anything to be done that can change it. Sometimes it's something as simple as a different haircut, a couple new outfits, and maybe a change in makeup (if they so desire to wear it). Sometimes it's something a bit deeper, like body-image issues. This can be helped with proper diet and exercise (both if they feel too large and if they feel to small).
But, all in all, I think the best way to help someone feel pretty isn't in changing what they look like. It's in being there for them and being the best friend you can be. Most of the time, feeling 'pretty' comes from a base sense of self-confidence. Being supportive, loving, and friendly to someone can do a number on their self-worth, thus making them eventually feel better about themselves. Beauty standards don't matter as long as you feel good about yourself.
In the end, though, does it really matter? Once you feel confident in yourself and the way you look, who cares if you're 'pretty' or 'handsome'? I mean, sure, beautiful people get lots of attention for their looks, and they have a predisposition to be more well-liked by strangers... but almost all of my good, intelligent, funny, amazing friends aren't particularly stunning to look at. I love them all unconditionally, and I think they're all beautiful, but it really is the personality that counts. Someone might be the most beautiful person in the world, but that counts for nothing if they have no personality.
Appearances are important for your own self-confidence, but don't conform to what others want just because THEY want you to be different. Be you, and be amazing. Beauty will follow whether you like it or not.