Are retired airline pilots allowed and able to help in an emergency situation?
United 232 might be close to what you're thinking about: a United flight training instructor on board helped the pilots to land the aircraft, although he wasn't retired. But for this scenario to happen you need a fairly specific combination of circumstances:


The passengers have to know there's an emergency
Your father has to believe he could help significantly
The cabin crew have to decide it's worth passing your father's offer to the pilots
The pilots have to decide it's worth bringing another, unknown pilot into the situation


The basic question would be: is your father's help going to be important enough that it's worth increasing the communication and CRM workload for the crew? That's not necessarily an easy decision to make, especially if time is very limited or the nature of the emergency isn't clear.

Being "allowed" to help isn't really a consideration here, by the way: pilots have very broad emergency authority to do whatever they think is best. 

Overall, it's certainly possible and it would be the pilot in command's decision to accept your father's offer or not. It's just very, very unlikely.