My wife passed away recently and she has a passcode on her phone that I don't know what it is. Is there a way to unlock the phone?

I'm sorry to hear that, and also that I can't give you more useful advice: but because the passcode is stored only on the phone it's not possible to retrieve it. Your only way forward is to erase the phone: if you have a backup you may be able to restore from that (though if it contains the passcode it might bring you back to square one). So you can unlock the phone but not retrieve its current contents.

Please see If you forgot the passcode on your iPhone, or your iPhone is disabled - Apple Support