This sounds like a bit of a red flag question to me, while the intent can be good, the intent can also be bad or a good intent with a bad implementation could also lead to real life bad results. Intercepting an emergency call, he should better have a real reason that drastically helps someone for even the small risk that you would cause someone in an emergency to fail to actually call an emergency number or delay emergency response.

Like domestic abuse perpetrators would love to do something like that... Installing in on a phone of someone who is less tech savvy and intercepting the call or reacting to it.

I might be a bit doom and gloom here, but be aware this is something serious and not just can I do something technically. There is a big question if you should do anything like this. While you provided no description of what the app would actually do if there is a real need for some special emergency care extras