Question: My dad is a pilot who flew over 15000 hours in 777.
Will he be able/allowed to help the crews if the same plane is in emergency?

Answer: 
[United 232](https://en.wikipedia.org/wiki/United_Airlines_Flight_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:

1. The passengers [have to know there's an emergency](https://aviation.stackexchange.com/q/14963/62)
2. Your father has to believe he could help significantly
3. The cabin crew have to decide it's worth passing your father's offer to the pilots
4. 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](https://en.wikipedia.org/wiki/Crew_resource_management) 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](https://aviation.stackexchange.com/q/18882/62) 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.
There are two questions here: will he be **able** and will he be **allowed**.

Will he be able to help? *Generally* yes. The retired pilot's ability to improve the situation depends on a few factors, such as how long has he been not flying, his mental and physical condition, any disparities between the model which he flown years ago and the current model, updated airport locations which he may not know etc.

Will he be allowed to help? Again, *generally* yes, but it is the decision of the pilot in command (or whoever is in charge if the PIC is unconscious). The cockpit only has so much space, and if it is decided that the pilots on duty can handle the situation already, there is no reason to bring onboard a retired pilot from the passenger cabin. Or, perhaps he is known to the flight crew as a past coworker of the airline, and the pilots may wish to listen to the view of an experienced captain.

To conclude: it depends, with a generally yes.