The restaurant has a right not to serve her.
But if the service is so bad, why does this lady return? I bet she’s some sort of nut.
At TGI Fridays the wait staff have to give three percent to the tip pool based upon their gross sales. So if their sales were $1000 for the night then they should have gotten around $150 for the night of which they have to give away $30 to the pool (goes to other staff). So when a customer runs up a bill of $90 because of drinks and so forth and then stiffs the waiter or waitress he is actually costing him/her money since their salary is $2.05 per hour.