The iPhone is quite old so only has iOS 9 and cannot be updated. Google calendar won't install
I’ve a radical idea, which I don’t think carries too much expense...
TALK TO EACH OTHER!!!
I've pretty much taken the approach suggested by a couple of people above. My wife and I each have our own gmail accounts for our main Android login, and another account that we share but don't use for email. With two gmail accounts added to the phone, apps such as Contacts and Calendar allow you to select which of your accounts to include, so we make sure both are selected. As you add new entries, you're then able to say which account they go to: personal or shared. I have Albion fixtures in my personal calendar, and things like holidays, birthdays and family visits in the shared one.
Schoolboy error. I can see this kind of situation arising:
Wife 'As we aren't doing anything next Saturday, I booked a weekend away in Edinburgh for that shopping trip you promised me....'
You: 'Er, but Albion are playing Liverpool in the final game of the season which decides whether we qualify for Europe and whether they win the league....'