I can't say I am the best to answer this question, but since no one else has I will give it a go. I mainly knit, but crochet sometimes. There is a company called knitpicks that has a website and offers nice quality yarns for very reasonable prices. For mittens, there are a lot of good choices with luxury fibers. For house slippers, sticking with straight wool would probably be the best way to go, since they need to hold up to more wear and tear.
If the crocheter in question likes sticking to acrylics, those can be found at a neighborhood craft store. Red Heart Super Saver comes in a lot of fun colors, but is not known for being "nice". Just walking around and feeling those can help you make a decision.