While I think generosity on the part of more senior scientists is a good thing, and should be encouraged, making this an obligation is flawed. How do you know what that person's obligations are?
Tenured profs @ conferences: adopt a "grad students & adjuncts don't pay" rule at dinner/bar. Some did this for student-me & I never forgot.
— Amy Lynch-Biniek (@amylynchbiniek) January 3, 2017
I post this in case any PI types out there don't know this is a thing. If you can pick up a check or pay more than your share, hey great. Good for you.
But nobody should expect it of you.