This is more of a judgment call. Catholics used to be categorically forbidden to go to Protestant services but now it's a bit looser.
Certainly, if it is posing a danger to his faith as a Catholic, he should not go. If not, what he is doing can probably be tolerated. I guess the question is:
Why specifically does he go?
- is it nostalgia?
- an affection for the beauty of the Anglican liturgy? or
- out of friendship with school chums, or what?
Perhaps he should bring this up with his confessor or pastor and see what he says. It is difficult for us at a distance to make a call.
You are correct that you need to fulfill your Sunday obligation first (I say this for the sake of other readers).
I'm not sure where you are, but there are Catholic churches that use a rite based on the Anglican liturgy. They are called Anglican Ordinariate parishes or Anglican Use parishes. You might want to look one up. Then he can have the riches of the Anglican liturgy and full communion with Catholicism.