I believe the answer is 24, for what it's now worth- although please correct me if I calculated this wrong:
Person A pots all the reds on the table, but fails to pot any colours subsequently:
Person A now has 15 points (15 x 1 for each red). There are currently 27 points now left available on the table (2+3+4+5+6+7 for each of the colours, yellow through to black.)
If person A then pots the yellow, (s)he has 17 points; 25 left on the table.
If person A pots the green, (s)he will have 20 points; 22 left on the table.
If person A now pots the brown, (s)he will have 24 points; leaving 18 on the table.
Thus, 24 points is the minimum I calculate person A to need, presuming that person B doesn't start looking for snookers.