Is it ok to be in touch with ex after marriage?

There's this guy I know... he is married to a woman. She is very average looking. She has told him about her past relationships...

He has now observed that she is still in touch through text/WhatsApp/calls with one of her ex who was already married back then. She sends him romantic messages/ YouTube shorts etc... and texts him late night/very early morning.

He says he doesn't really like her anyway but still married her.. he doesn't like her looks and now he doesn't have sex with her anymore.

What can he do? Whose fault is it?

It's his fault since he isn't providing her with sex and emotional/ romantic support
It's her fault and he should clearly ask her about it and know what's going on
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Is it ok to be in touch with ex after marriage?
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