I don't understand how it all evolved.
He agreed not to see her?
Who with, the mother?
If they made a decision like that, did they plan any further, like when the right time would be to introduce him again?
Personally I don't approve of a plan like this.
My husband is in the Army, too, and if we split up, it might not be easy to see his son on a regular basis, but I would definitely make sure that he would be able to stay in touch, and his son knew who he was.
I, however would not run after him, if he decided not to be part of his son's life.
That would be his loss.
And somehow, in your case it seems to me, that your friend's interest in his daughter is not all that big.
If in four years he's never even mentioned her to his girlfriend.
But, being an outsider, and only knowing part of the story, I might be totally wrong.
In any case, it is his decision, mention your concern to him ONCE, but do not interfere any further.