Thanks in advance for any advice. They are planning on a drink, burger, maybe dessert.
A tip is never required anywhere except, of course, where you agree to it contractually- by agreement. Sometimes you'll see on a menu words to the effect that "a gratuity of XX% will be added to bills of parties of eight or more...."
A tip (gratuity), as they call it, (I call a charge when required) other than the above, is always voluntary, that's why it's a tip.
I tip anytime service is adequate and food has been brought to me.
Better service gets a better tip out of me generally.
I sometimes put money in the tip jars at places like Ferch's and Chipotle so I suggest you do not hesitate to do so if
you feel it appropriate.
Of course food establishments are not the only place where tipping is done.