Personally, once you hit about oh 30 or more, age doesn't matter too much. Unless we are talking about maturity, personality, etc, then that is different.
If this guy is 17 and she's 20, there is a difference. The 20 year old is out on their own and probably in college. They both might have jobs, but the 17 year old probably still lives at home.
There are things that need to be looked at, too.
My husband is 9 years older than me. I don't really know if it seemed like a not so good idea for my mom and my brother, but they like my husband now.
My mom remarried and married a younger man...five years younger.
There are just so many different things that take into account when talking about 'dating', etc.
I don't think dating a siblings friend is a good idea (though I have that as a good story plot for a book lol). But hey...I have known people who do that and it works out really well.
Dating is like rolling a dice for most people...the only way you win is if you work at it...errrr roll the dice right lol. No relationship will just work by chance. There is more to it then that.