As a former HR administrator, what I can tell you is that turn over in the waitressing industry is something planned for. So, if you were to get an offer as a waitress before anything else comes along, go ahead and take it!

If the second interview works out and you get that job, well take it too. Then talk to the people at Friendly's and explain to them that you would like to maintain your employment with them but need to adjust your schedule. When they see that your "other job" is such a much better option for you, it will be their call to adjust your schedule to retain you, or to let you go. So, you put the ball at their court and what ever happens they will see you made a good decision when you accepted the "other job" and won't hold it against you.

gab