OP
The context of all of this is that you husband developed what he described as a friendship with a colleague. He found her attractive and she found him attractive. He developed mentionitis and spent more time than he should with her. Sharing things and giving her his personal number. She told him she wanted more than friendship. She acknowledged he was marrying you and she distanced herself from him. He then spent weeks stalking her on social media and being in a strange mood.
There are a lot of ways to describe his behaviour, a crush, an infatuation, an obsession or just too close for a friendship. This is also how we describe falling in love. I am very sorry if that cuts but from your last post there is quite a lot of head burying going on here.
The answer to your question is that it fine for a DH to buy an attractive colleague a coffee. It is not fine to buy one for someone for whom you have inappropriate feelings.
The gist of the last post was that you and he minimised what went on here. He isn’t facing up to his feelings or behaviour and you kept reducing things to individual acts. I accept you believe he loves you and your son but that wouldn’t have stopped him being able to fall in love.
He could have stopped that but he didn’t. He found excuses and he minimised what he was doing.
Fundamentally you don’t trust him and rightly so.
But it is no life if you are checking his phone and forensically checking how he spends his money. Where will it stop for you ?
My advice is for you both to wake up to the reality of what happened here otherwise it will eat at both of you.