The capital gains tax exemption for the sale of a primary residence can be used every 2 years. The basic rule is that it must have been a primary residence for 2 of the past 5 years. The capital gains exemption is $250K for a single person or $500K for a married couple filing jointly. The $250K and $500K exemption is profit, not the sale price of the house. If your dad has used it on the sale of a house in the past 2 years, your mom would not be eligible for the full amount.
Please feel free to contact me directly with any questions.
Jen Bowman, Broker Associate
Keller Williams Realty Cityside
p.s. This is not to be considered legal or tax advice, just speaking from experience. :)