Simple Answer:
No. Terminated employees cannot use remaining transit/parking balances.
Detailed Answer:
No. According to Treas. Reg. Sec. 1.132-9, Q/A-5:
May qualified transportation fringes be provided to individuals who are not employees?
A-5. An employer may provide qualified transportation fringes only to individuals who are currently employees of the employer at the time the qualified transportation fringe is provided.
http://www.gpo.gov/fdsys/pkg/CFR-2012-title26-vol2/pdf/CFR-2012-title26-vol2-sec1-132-9.pdf