My sister has mid-back length hair and she always blowdries her hair to help straighten it. She was wondering if it is okay to use a cool setting on the hair dryer. any advice?

Generally speaking, using a hair dryer is discouraged.  However, if used on a cool setting, and the hair is treated gently, the damage can be minimized.  I'd suggest regular conditioning and watching for and trimming split ends, if dryer use is continued.

There might be other options for handling her hair depending on the type of hair she has.  There are a number of suggestions listed on this site.... maybe your sister should come and check us out.  :D

I am guilty of using the hairdryer once in awhile, I'll admit, but with my schedule and limited time, sometimes I have too.  When I do I always, always, always use it on the cool setting & I also use a long-fingered diffuser to help minimize the impact on my hair.  I also only use it to get part of the dampness out, never until complete dryness, and I always follow up with a leave in moisturizer like jojoba oil.  Since making the switch to cool air only, there is definitely a difference in the health of my hair, although I do strongly feel that letting hair air dry whenever possible is ultimately best.

I agree.  Letting hair air dry is best ultimate solution to keeping your hair from any damage or slit ends.  I try to avoid blow drying as much as possible.